Quick Summary
- What this article covers: A comprehensive, working reference to every measuring unit used in mechanical engineering — base units, derived units, conversion factors, formulas, and the cross-system traps that cause real-world failures.
- Why it matters: A single misread unit can destroy a gearbox, crash a prototype, or void a contract. Fluency in both SI and English systems is no longer optional — it's a survival skill.
- Key insight: The SI system is coherent — no conversion factors between derived units. The English system is not. Understanding this single distinction prevents the majority of mechanical unit errors.
- Who this is for: Mechanical engineers, machinists, manufacturing professionals, students, technical buyers, and anyone who reads drawings, specs, or equipment manuals across measurement systems.
Introduction: The Düsseldorf Incident
Marco Reyes had been a mechanical engineer for eleven years. He'd designed hydraulic presses, calculated bearing loads, and specified motors for conveyor systems across three continents. He was good.
Then came the Düsseldorf project.
A German client commissioned a custom braking system. Marco's team in North America designed it using pound-force, foot-pounds, and horsepower. The manufacturing partner in Germany worked in newtons, joules, and kilowatts. Somewhere between Marco's design calculations and the German shop floor, a pressure value of 6,895 pascals was entered as 6,895 pounds per square inch — a factor of nearly 6,895× too high.
The prototype brake assembly failed catastrophically during testing. No one was hurt. The company lost three months and a significant sum in rework costs.
Marco didn't make a math error. He made a measurement-system error — and it's the kind of mistake that has haunted engineering since the industrial revolution. The Mars Climate Orbiter, the Gimli Glider, the Hubble primary mirror: history's most expensive engineering failures all share a common thread of unit confusion.
This guide exists so it never happens to you.
Core Concept: Two Worlds That Refuse to Merge
If you work in mechanics — designing machines, manufacturing parts, maintaining equipment, or studying engineering — you live in a world split between two measurement systems:
- The International System of Units (SI) — used by the vast majority of the world's industrialized nations and effectively universal in scientific publishing.
- The English (Imperial / US Customary) System — still deeply embedded in North American manufacturing, aerospace, oil and gas, construction, and decades of legacy equipment.
Neither system is going away. Drawings from the 1960s still live on shop floors. Equipment manuals arrive in pounds-force while CAD software defaults to newtons. International supply chains routinely mix both in a single bill of materials.
Your ability to fluently navigate both systems — and convert between them with precision — is the foundation everything else in this guide is built on.
The Seven Pillars: SI Base Units for Mechanics
Before you can master derived units like force, pressure, and power, you need to understand where they come from. The SI system is built on seven base units, adopted by the Conférence Générale des Poids et Mesures (CGPM).
Here are the ones that matter most in mechanics:
| Physical Quantity | SI Unit | Symbol | Definition |
|---|---|---|---|
| Length | meter | m | Distance traveled by light in vacuum in 1/299,792,458 of a second |
| Mass | kilogram | kg | Mass of the international prototype at the Bureau International des Poids et Mesures (BIPM), Sèvres, France |
| Time | second | s | Duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of cesium-133 |
| Temperature | kelvin | K | Fraction 1/273.16 of the thermodynamic temperature of the triple point of water |
| Amount of Substance | mole | mol | Amount of substance containing as many elementary entities as there are atoms in 0.012 kg of carbon-12 |
Why This Matters to You
Every mechanical quantity you'll ever calculate — force, torque, pressure, energy, power, viscosity, momentum — derives from these base units. When you understand the derivation chain, unit conversions stop being memorization and become logical deduction.
The flow looks like this:
Length, Mass, Time (base units)
↓
Velocity, Acceleration (kinematic derivatives)
↓
Force = Mass × Acceleration
↓
Pressure (Force ÷ Area) Energy (Force × Distance) Torque (Force × Distance)
↓ ↓
Power (Energy ÷ Time)
Master the chain and the rest follows.
The Inciting Distinction: Mass vs. Weight vs. Force
Here is where most confusion begins — and where Marco's Düsseldorf project went sideways.
The Critical Distinction
- Mass is the amount of matter in an object. It does not change whether you are on Earth, the Moon, or floating in space. SI unit: kilogram (kg).
- Weight is the force of gravity acting on that mass. It does change with location. SI unit: newton (N).
- Force is any interaction that changes the motion of an object. SI unit: newton (N).
A 10 kg object has a mass of 10 kg on Earth, on the Moon, and on Mars. Its weight, however, is about 98 N on Earth, 16 N on the Moon, and 37 N on Mars. Confusing these is the single most common origin of mechanical unit errors.
Newton's Second Law — The Origin of Force Units
The defining equation of mechanics:
F = m × a
Where:
- F = force (newtons)
- m = mass (kilograms)
- a = acceleration (meters per second squared)
The newton is defined as: the force which, when applied to a mass of one kilogram, gives it an acceleration of one meter per second squared.
1 N = 1 kg · m/s²
This definition is independent of gravitational acceleration (g). That single fact is what makes the SI system coherent — and what makes the English system a minefield.
The English System Trap: Pound-Mass vs. Pound-Force
In the English system, the word "pound" is used for both mass and force, which is the source of endless confusion:
- Pound-mass (lbm): a unit of mass — 0.4536 kg
- Pound-force (lbf): a unit of force — the gravitational force on one pound-mass at standard gravity
The "standard gravitational acceleration" is defined by international agreement as:
g = 9.80665 m/s² = 32.174 ft/s²
This means:
1 lbf = 1 lbm × 32.174 ft/s²
To make Newton's second law work in English units without carrying the factor of 32.174 around, engineers introduced the slug as a coherent unit of mass:
1 slug = 32.174 lbm
A force of 1 lbf accelerates 1 slug at 1 ft/s². But in real-world American engineering practice, mass is almost always quoted in pound-mass, which means the factor of 32.174 always lurks in the background.
The Kilogram-Force Problem
A similar issue existed in older metric practice. The kilogram-force (kgf) — also called the kilopond (kp) — was defined as the force exerted by standard gravity on one kilogram of mass:
1 kgf = 9.80665 N (exact)
In modern SI, the kilogram-force is not used. The newton replaced it entirely. But you will still find kgf on older equipment, legacy drawings, and in some European technical manuals — especially for pressure (kgf/cm²) and bolt tension.
The Master Force Conversion Table
| From | To | Multiply By |
|---|---|---|
| newton (N) | pound-force (lbf) | 0.2248089 |
| newton (N) | kilogram-force (kgf) | 0.1019716 |
| newton (N) | dyne | 100,000 (exact) |
| newton (N) | poundal | 7.23301 |
| pound-force (lbf) | newton (N) | 4.448222 |
| kilogram-force (kgf) | newton (N) | 9.806650 (exact) |
| dyne | newton (N) | 0.00001 (exact) |
| ounce-force | newton (N) | 0.2780139 |
| poundal | newton (N) | 0.1382550 |
Pro tip: In dynamics problems (moving and accelerating systems), the factor g disappears from SI calculations because force is defined independently of gravity. In statics problems (dead loads, hanging weights), g reappears because you need to convert from mass (kg) to weight force (N) using W = m × g.
Deep Dive: Every Mechanical Unit You'll Use
This is the working core of the guide — every mechanical quantity you'll encounter, organized, defined, and cross-referenced between SI and English systems.
Velocity and Speed
Velocity measures the rate of change of position. Speed is the scalar (direction-independent) equivalent.
SI Unit: meter per second (m/s)
| From | To | Multiply By |
|---|---|---|
| meter/second | foot/second | 3.280840 |
| meter/second | foot/minute | 196.8504 |
| meter/minute | foot/minute | 3.280840 |
| kilometer/hour | mile/hour | 0.6213712 |
| mile/hour | kilometer/hour | 1.609344 (exact) |
| mile/hour | feet/second | 1.466667 |
| millimeter/minute | inch/minute | 0.03937008 |
Common equivalents to memorize:
- 1 mile/hour = 88 feet/minute
- 1 knot (international) = 1.151 miles/hour
- 60 mph ≈ 26.82 m/s ≈ 96.56 km/h
Acceleration
Acceleration measures the rate of change of velocity.
SI Unit: meter per second squared (m/s²)
| From | To | Multiply By |
|---|---|---|
| meter/second² | foot/second² | 3.280840 |
| meter/second² | inch/second² | 39.37008 |
| inch/second² | meter/second² | 0.0254 (exact) |
Standard gravitational acceleration:
g = 9.80665 m/s² = 32.174 ft/s² = 981 cm/s²
This value is an international convention, not a local measurement. True local g varies by latitude, altitude, and underlying geology — roughly 9.78 m/s² at the equator to 9.83 m/s² at the poles.
Torque (Moment of Force)
Torque is the rotational equivalent of linear force — a force applied at a distance from a pivot.
SI Unit: newton-meter (N·m)
τ = F × d
Where F = force and d = perpendicular distance from the axis of rotation.
| From | To | Multiply By |
|---|---|---|
| newton-meter (N·m) | pound-force-foot (lbf·ft) | 0.7375621 |
| newton-meter (N·m) | pound-force-inch (lbf·in) | 8.850748 |
| pound-force-foot (lbf·ft) | newton-meter (N·m) | 1.355818 |
| pound-force-inch (lbf·in) | newton-meter (N·m) | 0.1129848 |
Pound-Inches to Newton-Meters Quick Reference
| lbf·in | N·m | lbf·in | N·m | lbf·in | N·m |
|---|---|---|---|---|---|
| 1 | 0.113 | 10 | 1.130 | 100 | 11.298 |
| 2 | 0.226 | 20 | 2.260 | 200 | 22.597 |
| 3 | 0.339 | 30 | 3.390 | 300 | 33.895 |
| 4 | 0.452 | 40 | 4.519 | 400 | 45.194 |
| 5 | 0.565 | 50 | 5.649 | 500 | 56.492 |
| 6 | 0.678 | 60 | 6.779 | 600 | 67.791 |
| 7 | 0.791 | 70 | 7.909 | 700 | 79.089 |
| 8 | 0.904 | 80 | 9.039 | 800 | 90.388 |
| 9 | 1.017 | 90 | 10.169 | 900 | 101.686 |
| 10 | 1.130 | 100 | 11.298 | 1000 | 112.985 |
Running Torque Formula
In motor-driven systems, the running torque Tr is:
Tr = (HP × 5,250) ÷ N
Where:
- HP = horsepower being supplied
- N = running speed in RPM
- 5,250 = a combined constant that converts horsepower to foot-pounds per minute and work per revolution into torque (in lbf·ft)
Pressure and Stress
Pressure is force per unit area — and one of the most frequently converted quantities in all of mechanical engineering. The Düsseldorf incident at the start of this guide was a pressure error.
SI Unit: pascal (Pa) = newton per meter squared (N/m²)
P = F ÷ A
The Pressure Unit Landscape
| Unit | Symbol | Equivalent |
|---|---|---|
| Pascal | Pa | 1 N/m² |
| Kilopascal | kPa | 1,000 Pa |
| Megapascal | MPa | 1,000,000 Pa |
| Bar | bar | 100,000 Pa (exact) |
| Standard Atmosphere | atm | 101,325 Pa (exact) |
| Pounds per square inch | psi (lbf/in²) | 6,894.757 Pa |
Critical Pressure Conversions
| From | To | Multiply By |
|---|---|---|
| psi (lb/in²) | kilopascal (kPa) | 6.894757 |
| psi (lb/in²) | bar | 0.06894757 |
| psi (lb/in²) | kg/cm² | 0.07030697 |
| psi (lb/in²) | N/mm² (MPa) | 0.006894757 |
| kilopascal | psi | 0.1450377 |
| bar | psi | 14.50377 |
| kg/cm² | psi | 14.22334 |
| atmosphere | psi | 14.6959 |
| atmosphere | pascal | 101,325 (exact) |
| kg/m² | pascal | 9.806650 (exact) |
| lb/ft² | pascal | 47.88026 |
PSI to Kilopascals Quick Reference
| psi | kPa | psi | kPa | psi | kPa |
|---|---|---|---|---|---|
| 1 | 6.895 | 10 | 68.948 | 100 | 689.476 |
| 2 | 13.790 | 20 | 137.895 | 200 | 1,378.951 |
| 3 | 20.684 | 30 | 206.843 | 300 | 2,068.427 |
| 4 | 27.579 | 40 | 275.790 | 400 | 2,757.903 |
| 5 | 34.474 | 50 | 344.738 | 500 | 3,447.379 |
| 10 | 68.948 | 100 | 689.476 | 1,000 | 6,894.757 |
Pressure From Fluid Columns
These relationships are essential in hydraulics and pneumatics:
- 1 foot of water at 62°F = 62.355 lb/ft² = 0.4335 lb/in²
- 1 inch of mercury at 62°F = 1.132 feet of water = 0.4898 lb/in²
- 1 atmosphere = 14.6959 psi = 29.92 inches of mercury = 33.90 feet of water
Editorial note: Material yield strength is almost always expressed in MPa in modern engineering. A typical mild steel yields around 250 MPa; high-strength alloy steels at 800–1,400 MPa; aerospace titanium near 900 MPa. Internalize the MPa scale — it is the lingua franca of mechanical design.
Energy and Work
Work is force applied over a distance. Energy is the capacity to do work. They share units.
SI Unit: joule (J) = newton-meter (N·m) = kg·m²/s²
W = F × d
| From | To | Multiply By |
|---|---|---|
| joule (J) | foot-pound (ft·lb) | 0.7375621 |
| joule (J) | Btu (International Table) | 0.0009478170 |
| joule (J) | calorie (mean) | 0.2386623 |
| foot-pound | joule | 1.355818 |
| Btu (International Table) | joule | 1,055.056 |
| calorie (mean) | joule | 4.19002 |
| foot-poundal | joule | 0.04214011 |
| watt-hour | joule | 3,600 (exact) |
Foot-Pounds to Joules Quick Reference
| ft·lb | J | ft·lb | J | ft·lb | J |
|---|---|---|---|---|---|
| 1 | 1.356 | 10 | 13.558 | 100 | 135.582 |
| 2 | 2.712 | 20 | 27.116 | 200 | 271.164 |
| 3 | 4.067 | 30 | 40.675 | 300 | 406.745 |
| 4 | 5.423 | 40 | 54.233 | 400 | 542.327 |
| 5 | 6.779 | 50 | 67.791 | 500 | 677.909 |
The Mechanical Equivalent of Heat
One of the most important constants in thermomechanics:
1 Btu = 778 foot‑pounds = 1,055 joules
The number 778 ft·lb per Btu is known as Joule's mechanical equivalent of heat. It bridges mechanical work and thermal energy, and it is the historical foundation of thermodynamics as a discipline.
Power
Power is the rate of doing work — energy transferred per unit time.
SI Unit: watt (W) = joule per second (J/s) = kg·m²/s³
P = W ÷ t
Power Equivalents — The Complete Picture
| Relationship | Value |
|---|---|
| 1 horsepower | 746 W = 0.746 kW |
| 1 horsepower | 33,000 ft·lb/min = 550 ft·lb/s |
| 1 horsepower | 42.4 Btu/min |
| 1 kilowatt | 1,000 W = 1.341 hp |
| 1 kilowatt | 44,200 ft·lb/min = 737.6 ft·lb/s |
| 1 kilowatt | 3,415 Btu/hour |
| 1 watt | 1 J/s = 0.00134 hp |
| 1 watt | 3.42 Btu/hour = 44.22 ft·lb/min |
Power Conversion Factors
| From | To | Multiply By |
|---|---|---|
| horsepower (550 ft·lb/s) | watt | 745.6999 |
| horsepower (550 ft·lb/s) | kilowatt | 0.7456999 |
| horsepower (metric) | watt | 735.499 |
| horsepower (electric) | watt | 746.0 (exact) |
| kilowatt | horsepower | 1.341022 |
| ft·lb/min | watt | 0.02259697 |
| ft·lb/hour | watt | 0.0003766161 |
| Btu/hour | watt | 0.2930711 |
Warning: There are multiple definitions of horsepower. Mechanical horsepower (550 ft·lb/s), metric horsepower (75 kgf·m/s = 735.499 W), electric horsepower (746 W exactly), and boiler horsepower (≈9,810 W) are all different values. Always confirm which horsepower you're working with — especially on European motor nameplates, which typically use metric horsepower.
Horsepower to Kilowatts Quick Reference
Based on 1 hp = 550 ft·lb/s = 0.7457 kW
| hp | kW | hp | kW | hp | kW |
|---|---|---|---|---|---|
| 1 | 0.746 | 10 | 7.5 | 100 | 74.6 |
| 2 | 1.491 | 20 | 14.9 | 200 | 149.1 |
| 3 | 2.237 | 30 | 22.4 | 500 | 372.8 |
| 5 | 3.729 | 50 | 37.3 | 1,000 | 745.7 |
Horsepower Formulas for Rotating Machinery
For Prony brakes and dynamometers:
HP = (2π × L × P × N) ÷ 33,000
Simplified approximation:
HP ≈ (N × P) ÷ 1,000
Where N = RPM, P = tangential force at the brake rim (lbf), L = lever arm length (ft).
Momentum
Momentum is the product of mass and velocity — a measure of an object's tendency to keep moving.
SI Unit: kilogram-meter per second (kg·m/s)
p = m × v
| From | To | Multiply By |
|---|---|---|
| kg·m/s | lb·ft/s | 7.233011 |
| kg·m/s | lb·in/s | 86.79614 |
| lb·ft/s | kg·m/s | 0.1382550 |
| lb·in/s | kg·m/s | 0.01152125 |
Moment of Inertia and Section Modulus
These properties describe how an object's mass or cross-section is distributed relative to an axis — critical for structural analysis, beam design, and rotating machinery.
Moment of Inertia (Mass)
| From | To | Multiply By |
|---|---|---|
| kg·m² | lb·ft² | 23.73036 |
| kg·m² | lb·in² | 3,417.171 |
| lb·ft² | kg·m² | 0.04214011 |
| lb·in² | kg·m² | 0.0002926397 |
Moment of Section (Area)
| From | To | Multiply By |
|---|---|---|
| meter⁴ | foot⁴ | 115.8618 |
| centimeter⁴ | inch⁴ | 0.02402510 |
| foot⁴ | meter⁴ | 0.008630975 |
| inch⁴ | centimeter⁴ | 41.62314 |
Section Modulus
| From | To | Multiply By |
|---|---|---|
| meter³ | foot³ | 35.31466 |
| meter³ | inch³ | 61,023.76 |
| foot³ | meter³ | 0.02831685 |
| inch³ | meter³ | 0.00001638706 |
Viscosity
Viscosity measures a fluid's resistance to shear — its "internal friction." Foundational for lubrication engineering, hydraulic system design, and any application involving fluid flow.
Dynamic (Absolute) Viscosity
SI Unit: pascal-second (Pa·s) = newton-second per meter squared (N·s/m²)
| From | To | Multiply By |
|---|---|---|
| pascal-second | centipoise (cP) | 1,000 (exact) |
| pascal-second | poise | 10 (exact) |
| poise | pascal-second | 0.1 (exact) |
Definition: one poise is the viscosity of a fluid requiring one dyne of force to move a surface of one square centimeter at a speed of one centimeter per second, with the surfaces one centimeter apart.
One reyn (English unit) = 68,950 poises.
Kinematic Viscosity
SI Unit: meter squared per second (m²/s)
| From | To | Multiply By |
|---|---|---|
| m²/s | centistoke (cSt) | 1,000,000 (exact) |
| m²/s | stoke (St) | 10,000 (exact) |
| stoke | m²/s | 0.0001 (exact) |
Relationship: kinematic viscosity = dynamic viscosity ÷ density
Viscosity-Pressure Relationship
For lubrication of highly loaded contacts — bearings, gears, cams — viscosity increases dramatically with pressure. The Barus equation describes this exponential rise:
ηₚ = η₀ × e^(α × p)
Where:
- η<sub>p</sub> = viscosity at pressure p (N·s/m²)
- η<sub>0</sub> = viscosity at atmospheric pressure (N·s/m²)
- α = pressure-viscosity coefficient (m²/N)
- p = pressure (N/m²)
| Lubricant | Dynamic Viscosity η₀ (× 10⁻³ N·s/m²) | Pressure-Viscosity Coefficient α (× 10⁻³ m²/N) |
|---|---|---|
| Light machine oil | 45 | 28 |
| Heavy machine oil | 153 | 23.7 |
| Cylinder oil | 810 | 34 |
| Spindle oil | 18.6 | 20 |
| Castor oil | 360 | 15.9 |
| Glycerol | 535 | 5.9 |
Temperature
Essential for thermal expansion calculations, heat treatment specifications, and material property lookups.
Conversion Formulas
°C = (°F − 32) × 5 ÷ 9
°F = (°C × 9 ÷ 5) + 32
K = °C + 273.16
°R = °F + 459.7
Where:
- K = Kelvin (absolute temperature, SI)
- °R = Rankine (absolute temperature, English)
Absolute Zero
The theoretical minimum temperature — where molecular motion ceases:
−273.16°C = −459.7°F = 0 K = 0°R
Thermal Energy Units
| Relationship | Value |
|---|---|
| 1 Btu | Heat to raise 1 lb of water by 1°F |
| 1 kilogram calorie | Heat to raise 1 kg of water by 1°C |
| 1 kilogram calorie | 3.968 Btu = 1,000 gram calories |
| 1 kilojoule | 0.9478 Btu |
The Transformation: The SI Coherent System
Here is the breakthrough that changed everything for Marco — and will change everything for you.
The SI system is coherent. That means the product or quotient of any two unit quantities in the system is automatically the unit of the resultant quantity. No conversion factors needed.
Watch this chain:
| Quantity | Derivation | SI Unit |
|---|---|---|
| Velocity | length ÷ time | m/s |
| Acceleration | velocity ÷ time | m/s² |
| Force | mass × acceleration | kg·m/s² = newton (N) |
| Pressure | force ÷ area | N/m² = pascal (Pa) |
| Work / Energy | force × distance | N·m = joule (J) |
| Power | energy ÷ time | J/s = watt (W) |
| Momentum | mass × velocity | kg·m/s |
| Torque | force × distance | N·m |
| Dynamic Viscosity | pressure × time | Pa·s |
This is the magic of SI: every derived unit is a clean product or quotient of base units. No 32.174. No 5,250. No 33,000. The dimensional analysis "just works."
The Complete SI Derived Units Table for Mechanics
| Physical Quantity | SI Unit | Formula |
|---|---|---|
| Area | square meter | m² |
| Volume | cubic meter | m³ |
| Frequency | hertz (Hz) | 1/s |
| Density | kg per cubic meter | kg/m³ |
| Velocity | meter per second | m/s |
| Angular Velocity | radian per second | rad/s |
| Acceleration | meter per second squared | m/s² |
| Angular Acceleration | radian per second squared | rad/s² |
| Pressure | pascal (Pa) | N/m² |
| Surface Tension | newton per meter | N/m |
| Dynamic Viscosity | pascal-second | Pa·s = N·s/m² |
| Kinematic Viscosity | meter squared per second | m²/s |
| Thermal Conductivity | watt per meter-kelvin | W/(m·K) |
SI Prefixes — The Scaling System
The SI system uses decimal prefixes to scale any unit by powers of 10:
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| tera | T | 10¹² | 1 TN = 1,000,000,000,000 N |
| giga | G | 10⁹ | 1 GPa = 1,000,000,000 Pa |
| mega | M | 10⁶ | 1 MPa = 1,000,000 Pa |
| kilo | k | 10³ | 1 kN = 1,000 N |
| hecto | h | 10² | 1 hPa = 100 Pa |
| deka | da | 10¹ | 1 daN = 10 N |
| — | — | 10⁰ | 1 N = 1 N |
| deci | d | 10⁻¹ | 1 dN = 0.1 N |
| centi | c | 10⁻² | 1 cm = 0.01 m |
| milli | m | 10⁻³ | 1 mN = 0.001 N |
| micro | µ | 10⁻⁶ | 1 µm = 0.000001 m |
| nano | n | 10⁻⁹ | 1 nm = 0.000000001 m |
| pico | p | 10⁻¹² | 1 pN = 0.000000000001 N |
Key insight: In mechanics, the most commonly used prefixes are kilo (kN, kPa, kW), mega (MPa, MW), and milli (mm, mN). Material stress is almost always expressed in MPa. Motor power is expressed in kW. Dimensions are in mm. Internalize these three defaults and 90% of mechanical SI work becomes intuitive.
Historical Systems: Know Them, Don't Fear Them
You will encounter legacy units on old drawings, in equipment manuals, and in textbooks from earlier eras. Understanding their lineage prevents misreading.
The CGS System (Centimeter-Gram-Second)
Before SI, the CGS system dominated science. You will still encounter:
| CGS Quantity | CGS Unit | SI Equivalent |
|---|---|---|
| Force | dyne | 1 dyne = 0.00001 N |
| Work / Energy | erg | 1 erg = 1 dyne·cm = 10⁻⁷ J |
| Power | — | 1 watt = 10,000,000 ergs/s |
| Viscosity | poise | 1 poise = 0.1 Pa·s |
| Kinematic Viscosity | stoke | 1 stoke = 10⁻⁴ m²/s |
| Gravitational acceleration | — | 981 cm/s² (at Paris) |
The MKS and MKSA Systems
Professor G. Giorgi proposed the MKS (meter-kilogram-second) system in 1902. In 1950, the International Electrotechnical Commission adopted the ampere as the fourth base unit, creating the MKSA system — the direct predecessor of modern SI.
The "Technical" Metric System
This system used the meter, the kilogram-force, and the second. Widely used in European engineering, it suffered from a fundamental flaw: its unit of force (kgf) varied with local gravitational acceleration. The standard value g = 9.81 m/s² was adopted by international agreement to minimize this variation — but it remained a system whose force unit depended on a constant of nature, which is exactly the problem SI was designed to solve.
Mass and Weight Conversions: The Complete Reference
Pounds to Kilograms
1 pound (avoirdupois) = 0.4535924 kilogram
| lb | kg | lb | kg | lb | kg |
|---|---|---|---|---|---|
| 1 | 0.454 | 10 | 4.536 | 100 | 45.359 |
| 2 | 0.907 | 20 | 9.072 | 200 | 90.718 |
| 3 | 1.361 | 30 | 13.608 | 500 | 226.796 |
| 5 | 2.268 | 50 | 22.680 | 1,000 | 453.592 |
| 10 | 4.536 | 100 | 45.359 | 5,000 | 2,267.962 |
Kilograms to Pounds
1 kilogram = 2.204622 pounds
| kg | lb | kg | lb | kg | lb |
|---|---|---|---|---|---|
| 1 | 2.205 | 10 | 22.046 | 100 | 220.462 |
| 2 | 4.409 | 20 | 44.092 | 200 | 440.924 |
| 5 | 11.023 | 50 | 110.231 | 500 | 1,102.311 |
| 10 | 22.046 | 100 | 220.462 | 1,000 | 2,204.622 |
Other Mass Conversions
| From | To | Factor |
|---|---|---|
| ounce (avoirdupois) | gram | 28.34952 |
| ounce (troy) | gram | 31.10348 |
| slug | kilogram | 14.59390 |
| ton (short, 2,000 lb) | kilogram | 907.1847 |
| ton (long, 2,240 lb) | kilogram | 1,016.047 |
| ton (metric) | kilogram | 1,000 (exact) |
| grain | gram | 0.06479891 |
Newtons to Pounds-Force: The Conversion You'll Use Most
1 newton = 0.2248089 pound-force
| N | lbf | N | lbf | N | lbf |
|---|---|---|---|---|---|
| 1 | 0.225 | 10 | 2.248 | 100 | 22.481 |
| 2 | 0.450 | 20 | 4.496 | 200 | 44.962 |
| 3 | 0.674 | 30 | 6.744 | 300 | 67.443 |
| 4 | 0.899 | 40 | 8.992 | 400 | 89.924 |
| 5 | 1.124 | 50 | 11.240 | 500 | 112.404 |
Power and Heat Equivalents: The Definitive Reference
These relationships bridge mechanical, electrical, and thermal energy:
| Equivalence | Value |
|---|---|
| 1 hp-hour | 0.746 kWh = 2,545 Btu = 1,980,000 ft·lb |
| 1 kWh | 1.34 hp-hours = 3,415 Btu = 2,655,000 ft·lb = 3,600,000 J |
| 1 hp | 746 W = 33,000 ft·lb/min = 550 ft·lb/s = 42.4 Btu/min |
| 1 kW | 1.34 hp = 44,200 ft·lb/min = 3,415 Btu/hr |
| 1 W | 1 J/s = 3.42 Btu/hr = 44.22 ft·lb/min |
| 1 Btu | 1,055 J = 778 ft·lb = 0.000293 kWh |
| 1 ft·lb | 1.356 J = 0.001285 Btu |
| 1 joule | 1 W·s = 0.7376 ft·lb = 0.000948 Btu |
Engineering Abbreviations: The Quick Reference
When reading technical drawings, specifications, and handbooks, you'll encounter these standard abbreviations:
| Abbreviation | Meaning | Abbreviation | Meaning |
|---|---|---|---|
| abs | Absolute | hp | Horsepower |
| atm | Atmosphere | Hz | Hertz |
| Bhn | Brinell hardness number | J | Joule |
| Btu | British thermal unit | kg | Kilogram |
| cal | Calorie | kPa | Kilopascal |
| cgs | Centimeter-gram-second | kW | Kilowatt |
| cm | Centimeter | kWh | Kilowatt-hour |
| cP | Centipoise | lb | Pound |
| cSt | Centistoke | lbf | Pound-force |
| ft | Foot | m | Meter |
| ft·lb | Foot-pound | mks | Meter-kilogram-second |
| ft/s | Feet per second | mm | Millimeter |
| ft/min | Feet per minute | MPa | Megapascal |
| g | Gram | N | Newton |
| gal | Gallon | N·m | Newton-meter |
| hp | Horsepower | Pa | Pascal |
Best practice: Abbreviations should be used sparingly and only where their meaning is unambiguous. When in doubt, spell out the term. Never begin a sentence with a numeral followed by an abbreviation.
A Practical Method: How to Use Conversion Tables
When a value falls outside the range of a printed conversion table, combine values from the table. Because all conversion factors are linear, you can decompose any value and sum the converted parts.
Example 1: Convert 0.4476 inch to millimeters
| Component | Conversion |
|---|---|
| 0.4 in | = 10.16000 mm |
| 0.04 in | = 1.01600 mm |
| 0.007 in | = 0.17780 mm |
| 0.0006 in | = 0.01524 mm |
| 0.4476 in | = 11.36904 mm |
Example 2: Convert 84.9 mm to inches
| Component | Conversion |
|---|---|
| 80 mm | = 3.14961 in |
| 4 mm | = 0.15748 in |
| 0.9 mm | = 0.03543 in |
| 84.9 mm | = 3.34252 in |
This additive method is the analog precursor to a calculator — but it remains useful as a sanity check and as a fast mental approximation when you don't have a tool at hand.
Real-World Examples: When Units Decide the Outcome
Case 1: The Mars Climate Orbiter (1999)
NASA's $327M Mars Climate Orbiter disintegrated in the Martian atmosphere because one engineering team supplied thrust data in pound-force-seconds while the navigation software expected newton-seconds. The factor of 4.45 between the two units accumulated over months of trajectory correction and put the spacecraft 170 km below its intended orbital insertion altitude.
Lesson: unit conventions must be specified at every interface, not assumed.
Case 2: The Gimli Glider (1983)
Air Canada Flight 143 ran out of fuel mid-flight after ground crew calculated fuel load using pounds while the new Boeing 767's metric-system gauges required kilograms. The factor of 2.2 meant the aircraft was loaded with less than half the required fuel. The captain glided the airliner to a safe landing at a former RCAF base in Gimli, Manitoba.
Lesson: transitioning between unit systems is a known high-risk operation, even with trained professionals.
Case 3: A Routine Torque Spec Goes Wrong
A maintenance technician applies a torque of 75 lbf·ft to a critical bolt because the SI spec of 75 N·m was misread. The applied torque is 1.36× the specification (75 lbf·ft = 101.7 N·m). The bolt yields. A pump seal fails six months later.
Lesson: torque values without explicit units are radioactive. Always include them.
Common Mistakes and How to Prevent Them
| Mistake | Consequence | Prevention |
|---|---|---|
| Confusing lbm and lbf | Forces off by factor of 32.174 in dynamics | Use slug-foot-second or convert to SI for all dynamics |
| Treating "pound" as unambiguous | 50% chance of getting force vs. mass wrong | Always specify lbf or lbm explicitly |
| Mixing horsepower definitions | Motor sizing errors of 1–2% (mechanical vs. metric hp) | Verify hp definition on every nameplate |
| Using kgf/cm² as if it were pascal | Pressure off by factor of 98,066.5 | Convert legacy kgf/cm² to kPa or MPa immediately |
| Dropping units mid-calculation | Dimensional errors not caught until final check | Carry units through every step of every formula |
| Assuming "psi" means absolute | Gauge vs. absolute pressure confusion | Always specify psia or psig |
| Mixing SI prefixes (MPa vs. kPa) | Off by 1,000× — most dangerous in stress calcs | Standardize on MPa for stress, kPa for fluid pressure |
| Reading old drawings without unit context | Misinterpretation of 1960s-era specs | Always check the title block for unit system before reading values |
Expert Insights: Heuristics From the Field
1. Memorize the Five Most Frequent Conversions
You will use these constantly. Burn them into memory:
- 1 inch = 25.4 mm (exact)
- 1 lbf = 4.448 N
- 1 psi = 6.895 kPa
- 1 hp = 0.746 kW
- 1 ft·lb = 1.356 J ≈ 1.356 N·m
2. Use the SI Coherent Chain as a Sanity Check
If you're calculating power in watts, the units must reduce to kg·m²/s³. If they don't, something has gone wrong upstream. Dimensional analysis is the most underused debugging tool in mechanical engineering.
3. Convert to SI at the Earliest Possible Point
Once everything is in SI, no further conversions are required until you communicate results to a customer or partner. The SI system's coherence means dimensional analysis becomes trivial — and trivial dimensional analysis catches errors that would otherwise propagate through hundreds of calculations.
4. Annotate Drawings With Both Units When Crossing Borders
If a drawing will be read by both North American and European audiences, dual-dimension it. Modern CAD systems do this automatically. The cost of dual annotation is near zero. The cost of misreading is potentially catastrophic.
5. Treat Legacy Units as a Translation Problem
A 1965 drawing in psi and lbf·ft is not "wrong" — it's in a different language. Translate the drawing once, save the SI version, and work from that. Never calculate from mixed-unit source documents.
6. Beware of the Three-Letter Trap
kgf, lbf, ozf look like mass units because they share roots with kg, lb, oz. They are not. They are force units defined by standard gravity. Any time you see the suffix "-f," your dimensional radar should activate.
FAQ Section
What is the difference between mass and weight?
Mass is the amount of matter in an object (measured in kilograms or pound-mass). Weight is the gravitational force on that mass (measured in newtons or pound-force). Mass is invariant; weight changes with location. On the Moon, your mass is unchanged but your weight is roughly 1/6 of Earth's.
Why does mechanical engineering still use English units?
Legacy. The North American aerospace, oil and gas, and manufacturing industries built their tooling, drawings, and standards in English units before the global SI transition. Replacing decades of physical infrastructure and reference documentation is prohibitively expensive, so dual-system literacy remains a permanent requirement.
Is 1 psi equal to 1 N/m²?
No. 1 psi = 6,894.757 N/m² (pascals). The factor is significant — confusing the two by treating "psi" as equivalent to "kPa" was the root cause of the Düsseldorf incident at the start of this guide.
What's the difference between mechanical, metric, and electric horsepower?
Mechanical horsepower = 550 ft·lb/s = 745.6999 W. Metric horsepower = 75 kgf·m/s = 735.499 W. Electric horsepower = 746 W (exact). The differences are small (under 1.5%) but matter for precision motor sizing and contractual specifications.
Why is the kilogram-force not used in modern SI?
Because the kgf is defined by standard gravitational acceleration, which means its value implicitly depends on a constant of nature. SI was designed to be coherent and independent of local conditions — the newton, defined purely from mass and acceleration, replaced the kgf entirely.
What is a "coherent" unit system?
A unit system is coherent when derived units are products or quotients of base units with no numerical factors. SI is coherent: 1 N = 1 kg·m/s², 1 Pa = 1 N/m², 1 J = 1 N·m, 1 W = 1 J/s. The English system is not — it requires factors like 32.174, 5,250, and 33,000 in routine calculations.
How accurate are the conversion factors in this guide?
All factors are either marked exact (defined by international agreement, e.g., 1 inch = 25.4 mm) or carried to seven significant figures, which exceeds the precision required for virtually all engineering applications.
Should I learn the slug?
If you work in American dynamics calculations, yes. The slug is the coherent English unit of mass: 1 lbf accelerates 1 slug at 1 ft/s². It eliminates the factor of 32.174 in F = m × a. If you work primarily in SI, you can ignore the slug entirely.
What pressure unit should I use for material stress?
Megapascal (MPa). It is the universal modern convention. 1 MPa = 1 N/mm², which makes stress in MPa numerically equal to stress in N/mm² — a convenient identity for working with dimensions in millimeters.
How do I convert temperature differences vs. absolute temperatures?
Absolute conversions require the full formula (e.g., °F to °C requires subtracting 32 first). Temperature differences convert simply by the scale ratio: a difference of 5°F equals a difference of 2.78°C (since 1°F = 5/9 °C). Mixing these up is a common source of error in thermal expansion calculations.
Final Takeaways: Marco's Three-Part Protocol
After the Düsseldorf incident, Marco implemented a protocol that he now teaches every junior engineer who joins his team.
Rule 1: State Your Units — Every Time
Never write a number without its unit. Not on a napkin sketch. Not in an email. Not in a spreadsheet cell. "500" means nothing. "500 N" means everything. This is the single highest-leverage habit in engineering communication.
Rule 2: Convert at the Source
Don't convert in your head mid-calculation. Convert all inputs to one consistent system before you begin computing. Use SI for new designs. Use English only when legacy equipment demands it. Save the converted version and work from that exclusively.
Rule 3: Verify the Dimension Chain
After every calculation, check that your units cancel correctly:
Force = mass × acceleration = kg × (m/s²) = kg·m/s² = N ✓
If your final answer has unexpected units — or no units — stop and re-examine. Dimensional analysis catches more errors than any other debugging technique in mechanical engineering.
Your Next Step
Bookmark this guide. Print the conversion tables. Tape the force and pressure tables to your workstation. Save the SI prefix table to your phone.
Then ask yourself: where in your current workflow are you most vulnerable to a unit-conversion error?
Is it in torque specifications crossing between English and SI drawings? Pressure values on legacy hydraulic schematics? Power ratings on motors sourced from different countries? Material stress values on imported steel certificates?
Find that vulnerability. Close it. And share this reference with the next engineer who needs it.
Because the next Düsseldorf doesn't have to happen.
This guide draws from engineering reference standards including ANSI/ASME, ISO 1000, ISO 80000, ASTM SI10, and the International System of Units (SI) as defined by the CGPM. All conversion factors are exact or carried to the precision published in international standards. Formulas use generic notation for universal applicability across regions and decades.