Home Guides Engineering Unit Conversion Reference Engineering Fundamentals Engineering Unit Conversion Reference Complete reference for converting between engineering units: length, pressure, temperature, flow, power, energy, mass, and volume with quick-reference tables.
The International System of Units (SI) is the standard for engineering worldwide:
Quantity SI Unit Symbol Length meter m Mass kilogram kg Time second s Temperature Kelvin K Amount of substance mole mol Electric current Ampere A Luminous intensity candela cd
US customary (Imperial) units are still widely used in US industries, oil/gas, and some manufacturing. Engineers must be fluent in both systems.
From To Multiply by inches to mm mm 25.4 feet to meters m 0.3048 yards to meters m 0.9144 miles to km km 1.6093 mils (0.001 in) to mm mm 0.0254 Angstrom to m m 10⁻¹⁰
From To Multiply by square inches to mm² mm² 645.16 square feet to m² m² 0.0929 acres to m² m² 4046.9 hectares to m² m² 10,000 cubic inches to cm³ cm³ 16.387 cubic feet to m³ m³ 0.0283 gallons (US) to L L 3.785 gallons (Imperial) to L L 4.546 barrels (oil, bbl) to m³ m³ 0.159 barrels (oil) to gallons US gal 42
1 m³ = 1000 L = 264.2 US gallons = 6.29 barrels (oil)
From To Multiply by pounds (lbm) to kg kg 0.4536 ounces (oz) to g g 28.35 tons (short, 2000 lb) to kg kg 907.2 tonnes (metric, 1000 kg) to kg kg 1000 long tons (2240 lb) to kg kg 1016
Note: In engineering, weight = force = mass × gravity. A mass of 1 kg weighs 9.81 N (Newtons) on Earth. A 1-pound mass exerts 1 pound-force (lbf) under standard gravity.
Pressure = force per area. SI unit = Pascal (Pa = N/m²).
Unit SI Equivalent Common Use 1 Pascal 1 N/m² Very small; use kPa or MPa 1 bar 100,000 Pa (100 kPa) Standard industrial 1 atm 101,325 Pa Standard atmospheric 1 psi 6,894.8 Pa US customary 1 kgf/cm² (at) 98,067 Pa Metric technical atm 1 torr (mmHg) 133.3 Pa Vacuum measurements 1 in H₂O 248.8 Pa Low pressure/drafts 1 in Hg 3,386 Pa Barometric pressure 1 mm Hg 133.3 Pa Medical/lab
1 bar = 14.50 psi = 0.987 atm = 29.53 in Hg = 10.2 m H₂O = 750 torr
Scale Freezing Water Boiling Water Absolute Zero Celsius (°C) 0 100 -273.15 Fahrenheit (°F) 32 212 -459.67 Kelvin (K) 273.15 373.15 0 Rankine (°R) 491.67 671.67 0
°C = (°F − 32) × 5/9
°F = °C × 9/5 + 32
K = °C + 273.15
°R = °F + 459.67
From To Multiply by US gallons/min (gpm) to m³/h m³/h 0.227 US gpm to L/s L/s 0.0631 barrels/day to m³/h m³/h 0.00662 barrels/hour to m³/h m³/h 0.159 ft³/min (cfm) to m³/h m³/h 1.699 ft³/min (cfm) to L/s L/s 0.472 million gpd (MGD) to m³/h m³/h 157.7 L/min to m³/h m³/h 0.06
Gas flows are often referenced to standard conditions (1 atm, specified temperature):
Nm³/h : Normal cubic meters per hour (0°C, 1.013 bar abs)SCFM : Standard cubic feet per minute (60°F, 14.7 psia)
1 Nm³/h = 0.622 SCFM (approximately — depends on gas composition)
Convert actual flow to standard:
Qstd = Qactual × (Pactual /Pstd ) × (Tstd /Tactual )
From To Multiply by ft/s to m/s m/s 0.3048 ft/min to m/s m/s 0.00508 mph to km/h km/h 1.609 knots to m/s m/s 0.514 km/h to m/s m/s 0.2778
Quantity Unit SI Equivalent Force 1 lbf 4.448 N Force 1 kgf (kilopond) 9.807 N Force 1 dyne 10⁻⁵ N Torque 1 ft-lbf 1.356 N·m Torque 1 in-lbf 0.113 N·m Torque 1 kgf·m 9.807 N·m
Unit SI Equivalent Common Use 1 Watt 1 J/s SI standard 1 kilowatt (kW) 1000 W Electrical/mechanical 1 horsepower (hp) 745.7 W US mechanical power 1 metric hp (PS, CV) 735.5 W European metric hp 1 ft-lbf/s 1.356 W US customary 1 Btu/h 0.293 W Thermal (US) 1 ton of refrigeration 3,517 W (12,000 Btu/h) HVAC cooling 1 boiler hp 9,809 W Steam boilers
1 hp = 0.746 kW = 550 ft-lb/s = 2,545 Btu/h
Unit SI Equivalent Common Use 1 Joule 1 N·m SI standard 1 kilojoule (kJ) 1000 J 1 kilowatt-hour (kWh) 3.6 × 10⁶ J = 3.6 MJ Electrical billing 1 Btu 1055 J Thermal (US) 1 calorie (IT) 4.187 J Heat 1 food Calorie 4187 J = 4.187 kJ Nutrition (kilocalorie) 1 therm 105.5 MJ Natural gas billing 1 ft-lbf 1.356 J Mechanical work 1 kcal 4.187 kJ Thermal energy
1 kWh = 3,412 Btu = 860 kcal = 3.6 MJ
From To Multiply by lb/ft³ to kg/m³ kg/m³ 16.018 g/cm³ to kg/m³ kg/m³ 1000 lb/gal (US) to kg/m³ kg/m³ 119.8
Water density = 1000 kg/m³ = 62.4 lb/ft³ = 8.34 lb/gal
From To Multiply by centipoise (cP) to Pa·s Pa·s 0.001 poise to Pa·s Pa·s 0.1 lb/(ft·s) to Pa·s Pa·s 1.488
Water at 20°C = 1 cP = 0.001 Pa·s
From To Multiply by centistoke (cSt) to m²/s m²/s 10⁻⁶ ft²/s to m²/s m²/s 0.0929 Saybolt Seconds Universal (SSU) to cSt cSt ~0.216×SSU − 135 (for SSU > 100)
Water at 20°C = 1 cSt = 10⁻⁶ m²/s
Property Units Conversion Thermal conductivity Btu/(h·ft·°F) → W/(m·K) ×1.731 Heat transfer coefficient Btu/(h·ft²·°F) → W/(m²·K) ×5.678 Heat flux Btu/(h·ft²) → W/m² ×3.155 Specific heat Btu/(lb·°F) → kJ/(kg·K) ×4.187
Units R J/(mol·K) 8.314 kJ/(kmol·K) 8.314 ft-lbf/(lbmol·°R) 1545 Btu/(lbmol·°R) 1.986 m³·Pa/(mol·K) 8.314 L·atm/(mol·K) 0.08206
Gas MW (kg/kmol) R (J/kg·K) Air 28.97 287 Nitrogen 28.0 297 Oxygen 32.0 260 Methane 16.04 518 CO₂ 44.0 189 Hydrogen 2.02 4124 Steam 18.02 462
psi → bar : ÷ 14.5bar → psi : × 14.5°F → °C : (°F−32) × 5/9gpm → m³/h : × 0.227ft → m : × 0.305lb → kg : × 0.454hp → kW : × 0.746Btu → kJ : × 1.055in → mm : × 25.4mile → km : × 1.61
Engineering calculations require consistent units — always convert to SI (or one consistent system) before plugging into formulas. The SI system uses meters, kilograms, seconds, Kelvin, and Pascals. Critical conversion factors: 1 bar = 14.5 psi = 100 kPa, 1 m³ = 264 US gallons, °C = (°F − 32) × 5/9, 1 hp = 0.746 kW. Temperature differences use the 9/5 or 5/9 factor without offset. Always check units in formulas — unit errors cause some of the most expensive engineering mistakes.