Unit Conversion and SI Reference Tables
The reference that saves you a hundred searches
Every engineer, dozens of times a week, needs to go from psi to bar, from hp to kW, or from inches to millimetres. This page gathers the International System of Units (SI) and the exact conversion factors for every engineering quantity — plus embedded interactive converters that compute as you read. Bookmark it; you'll be back.
Every factor here is exact by definition or accurate to 7 significant figures. For sensitive design work, use the exact value, not a rounded one.
1. SI: the seven base units
Every other engineering unit is derived from these seven.
| Quantity | Unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
2. Common derived units
| Quantity | Unit | Symbol | In base units |
|---|---|---|---|
| Force | newton | N | kg·m/s² |
| Pressure | pascal | Pa | N/m² = kg/(m·s²) |
| Energy / work | joule | J | N·m = kg·m²/s² |
| Power | watt | W | J/s |
| Charge | coulomb | C | A·s |
| Voltage | volt | V | W/A |
| Resistance | ohm | Ω | V/A |
| Capacitance | farad | F | C/V |
| Frequency | hertz | Hz | 1/s |
3. Decimal (SI) prefixes
A wrong prefix is an error of a thousandfold or more. Know them cold.
| Prefix | Symbol | Factor | Prefix | Symbol | Factor | |
|---|---|---|---|---|---|---|
| tera | T | 10¹² | milli | m | 10⁻³ | |
| giga | G | 10⁹ | micro | µ | 10⁻⁶ | |
| mega | M | 10⁶ | nano | n | 10⁻⁹ | |
| kilo | k | 10³ | pico | p | 10⁻¹² | |
| hecto | h | 10² | femto | f | 10⁻¹⁵ | |
| deca | da | 10¹ | atto | a | 10⁻¹⁸ |
Added in 2022 — larger: ronna (R, 10²⁷) and quetta (Q, 10³⁰); smaller: ronto (r) and quecto (q).
4. Dimensional analysis: convert without error
The foolproof method is to multiply by "a factor equal to 1". Example: convert 72 km/h to m/s.
72 km/h × (1000 m ÷ 1 km) × (1 h ÷ 3600 s) = 20 m/s
The rule: arrange the fractions so the unwanted unit cancels (km on top with km on the bottom), leaving what you want. If the dimensions come out right, the arithmetic is right.
5. Length & distance
Base: metre (m).
| Unit | = metres |
|---|---|
| kilometre (km) | 1000 |
| centimetre (cm) | 0.01 |
| millimetre (mm) | 0.001 |
| mile (mi) | 1609.344 |
| yard (yd) | 0.9144 |
| foot (ft) | 0.3048 |
| inch (in) | 0.0254 |
| nautical mile (nmi) | 1852 |
convert-length
1 inch = 25.4 mmexactly, and1 foot = 12 inches = 304.8 mm.
6. Area
Base: square metre (m²). Note: land units differ across Arab countries.
| Unit | = square metres |
|---|---|
| square kilometre (km²) | 1,000,000 |
| hectare (ha) | 10,000 |
| dunam | 1000 |
| feddan (Egypt) | 4200.833 |
| acre | 4046.856 |
| square foot (ft²) | 0.092903 |
| square inch (in²) | 0.00064516 |
convert-area
Regional note: a dunam is 1000 m² in the Levant, while the Egyptian feddan is 4200.833 m² — neither equals the US acre (4046.856 m²). Never mix them in land contracts.
7. Volume & capacity
Base: cubic metre (m³), with the litre alongside (1 m³ = 1000 L).
| Unit | = cubic metres |
|---|---|
| litre (L) | 0.001 |
| millilitre / cm³ (mL) | 0.000001 |
| US gallon (gal US) | 0.003785412 |
| UK gallon (gal UK) | 0.00454609 |
| cubic foot (ft³) | 0.028316847 |
| oil barrel (bbl) | 0.158987295 |
convert-volume
The US gallon (3.785 L) is about 20% smaller than the UK gallon (4.546 L) — a common trap in imported pump specs.
8. Mass & weight
Base: kilogram (kg).
| Unit | = kilograms |
|---|---|
| tonne (t) | 1000 |
| gram (g) | 0.001 |
| pound (lb) | 0.45359237 |
| ounce (oz) | 0.028349523 |
| short ton (US) | 907.18474 |
| long ton (UK) | 1016.046909 |
convert-mass
Mass or weight? The kilogram is mass; weight is a force (newtons). A 1 kg mass weighs ≈ 9.81 N on Earth. In engineering we keep them strictly separate.
9. Temperature (an offset conversion)
Temperature is not proportional: it has a shifted zero, so you can't multiply by a simple factor — use the equations.
| From → To | Equation |
|---|---|
| Celsius → Fahrenheit | °F = °C × 9/5 + 32 |
| Fahrenheit → Celsius | °C = (°F − 32) × 5/9 |
| Celsius → Kelvin | K = °C + 273.15 |
| Kelvin → Celsius | °C = K − 273.15 |
| Fahrenheit → Rankine | °R = °F + 459.67 |
convert-temperature
Two anchors:
0 °C = 32 °F = 273.15 K, and−40 °C = −40 °Fexactly (where the two scales cross).
10. Pressure
Base: pascal (Pa). Standard atmosphere = 101,325 Pa.
| Unit | = pascals |
|---|---|
| kilopascal (kPa) | 1000 |
| bar | 100,000 |
| atmosphere (atm) | 101,325 |
| psi (lbf/in²) | 6894.757 |
| mmHg (torr) | 133.3224 |
| kgf/cm² | 98,066.5 |
convert-pressure
1 bar ≈ 14.5 psi ≈ 0.987 atm. Tyre pressure "in pounds" means psi.
11. Force
Base: newton (N).
| Unit | = newtons |
|---|---|
| kilonewton (kN) | 1000 |
| kilogram-force (kgf) | 9.80665 |
| pound-force (lbf) | 4.448222 |
| dyne (dyn) | 0.00001 |
| kip | 4448.222 |
convert-force
12. Torque
Base: newton-metre (N·m). Don't confuse it with energy despite the identical dimensions.
| Unit | = newton-metres |
|---|---|
| kgf·m | 9.80665 |
| kgf·cm | 0.0980665 |
| lbf·ft | 1.355818 |
| lbf·in | 0.112985 |
convert-torque
Bolt torque specs are usually N·m or lbf·ft:
1 lbf·ft ≈ 1.356 N·m.
13. Energy & work
Base: joule (J).
| Unit | = joules |
|---|---|
| kilojoule (kJ) | 1000 |
| kilowatt-hour (kWh) | 3,600,000 |
| calorie (cal) | 4.184 |
| kilocalorie (kcal) | 4184 |
| BTU | 1055.056 |
convert-energy
Your electricity bill is in kWh:
1 kWh = 3.6 MJ. A 2000 W appliance run for one hour uses 2 kWh.
14. Power
Base: watt (W). Mind the two kinds of "horsepower".
| Unit | = watts |
|---|---|
| kilowatt (kW) | 1000 |
| mechanical HP (hp) | 745.6999 |
| metric HP (PS) | 735.49875 |
| BTU/hour | 0.293071 |
| kcal/hour | 1.163 |
convert-power
Common trap: mechanical horsepower (745.7 W) is not metric HP / PS (735.5 W). European engine specs are often in PS.
15. Speed
Base: metre/second (m/s).
| Unit | = m/s |
|---|---|
| km/hour (km/h) | 0.277778 |
| mile/hour (mph) | 0.44704 |
| foot/second (ft/s) | 0.3048 |
| knot (kn) | 0.514444 |
| Mach (sea level) | 340.29 |
convert-speed
Quick trick:
m/s × 3.6 = km/h.
16. Frequency & rotational speed
Base: hertz (Hz).
| Unit | = hertz |
|---|---|
| kilohertz (kHz) | 1000 |
| megahertz (MHz) | 1,000,000 |
| gigahertz (GHz) | 1,000,000,000 |
| rev/minute (rpm) | 0.0166667 |
| radian/second (rad/s) | 0.159155 |
convert-frequency
60 rpm = 1 Hz, and1 Hz = 2π rad/s ≈ 6.283 rad/s.
17. Angles
Base: radian (rad). A full circle = 2π radians = 360°.
| Unit | = radians |
|---|---|
| degree (°) | 0.0174533 |
| gradian (gon) | 0.0157080 |
| full turn | 6.283185 |
| arcminute (′) | 0.000290888 |
| arcsecond (″) | 0.00000484814 |
convert-angle
180° = π radians, and90° = π/2 ≈ 1.5708 rad.
18. Time
Base: second (s).
| Unit | = seconds |
|---|---|
| minute (min) | 60 |
| hour (h) | 3600 |
| day | 86,400 |
| week | 604,800 |
| year (365.25 days) | 31,557,600 |
convert-time
19. Volumetric flow rate
Base: cubic metre/second (m³/s). Essential in HVAC and pumping.
| Unit | = m³/s |
|---|---|
| m³/hour (m³/h) | 0.000277778 |
| litre/second (L/s) | 0.001 |
| litre/minute (L/min) | 0.0000166667 |
| US gallon/minute (gpm) | 0.0000630902 |
| ft³/minute (CFM) | 0.000471947 |
convert-flow
20. Data storage (decimal vs binary)
Base: byte (B). A bit is 1/8 of a byte.
| Unit | = bytes | System |
|---|---|---|
| kilobyte (KB) | 1000 | decimal (10³) |
| megabyte (MB) | 1,000,000 | decimal (10⁶) |
| gigabyte (GB) | 1,000,000,000 | decimal (10⁹) |
| kibibyte (KiB) | 1024 | binary (2¹⁰) |
| mebibyte (MiB) | 1,048,576 | binary (2²⁰) |
| gibibyte (GiB) | 1,073,741,824 | binary (2³⁰) |
convert-data
Why does a hard drive look smaller? The maker sells it in decimal (1 TB = 10¹² bytes); the OS shows it in binary (1 TiB = 2⁴⁰ bytes) — so 1 TB appears as 0.909 TiB.
Going further
- Every converter above has its own standalone page in the Calculators section, working offline.
- See the Cable & Wire Tables and Electronics Reference Tables for more practical references.