SI Base Units
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.
Length
| 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 |
Area and Volume
| 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 |
Mass and Weight
| 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
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 |
Temperature
| 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 |
Conversions
Flow Rate
| 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 |
Standard/Gas Flows
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:
Velocity
| 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 |
Force and Torque
| 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 |
Power
| 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 |
Energy
| 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 |
Density and Viscosity
Density
| 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
Dynamic Viscosity
| 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
Kinematic Viscosity (ν = μ/ρ)
| 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
Thermal Conductivity and Heat Transfer
| 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 |
Gas Constants and Standard Conditions
Gas Constant R Values
| 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 |
Specific Gas Constants (Rspecific = Runiversal/M)
| 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 |
Quick Conversion Cheat Sheet
To Convert US to SI (Memorize These)
- psi → bar: ÷ 14.5
- bar → psi: × 14.5
- °F → °C: (°F−32) × 5/9
- gpm → m³/h: × 0.227
- ft → m: × 0.305
- lb → kg: × 0.454
- hp → kW: × 0.746
- Btu → kJ: × 1.055
- in → mm: × 25.4
- mile → km: × 1.61
Practical Conversion Examples
Worked conversions show how the tables apply to real engineering situations:
Example 1 — Pump discharge pressure. A pump spec reads 150 psi. Convert to bar and kPa: 150 ÷ 14.5 = 10.3 bar; 150 × 6.895 = 1034 kPa. The same pressure is often quoted as 150 psi, 10.3 bar, or 1.03 MPa — always convert before comparing vendor data.
Example 2 — Pipe flow rate. A fire water line delivers 800 gpm. Convert to m³/h and L/s: 800 × 0.227 = 182 m³/h; 800 × 0.0631 = 50.5 L/s. Use the Pipe Velocity Calculator to check the resulting velocity against erosion limits.
Example 3 — Heat exchanger duty. A heat exchanger duty is 2.5 MMBtu/h. Convert to kW: 2.5 × 293 = 733 kW. If the same duty were quoted in tons of refrigeration, 733 ÷ 3.517 = 208 TR.
Example 4 — Temperature difference. A steam system operates at 180°C with a 20 K temperature drop. The ΔT is 20 K (no offset needed for differences). For absolute temperatures: 180°C = 356°F = 453 K.
Example 5 — Gas density from standard conditions. Standard air is 1.225 kg/m³ at ISO. At 7 bar gauge and 40°C: ρ = 1.225 × (8.013/1.013) × (288.15/313.15) = 8.9 kg/m³. See Compressible Flow Basics and the Density Calculator for this calculation.
Example 6 — Hydraulic power. A pump moves 500 L/min at 100 m head. Hydraulic power = ρgQH = 1000 × 9.81 × (0.5/60) × 100 = 8.18 kW. With 70% efficiency, motor input ≈ 11.7 kW — confirm with the Pump Power Calculator.
Frequently Asked Questions
What is the easiest way to convert psi to bar? Divide psi by 14.5 (exact: 14.5038). 100 psi ≈ 6.9 bar. For site estimates, psi ÷ 15 is accurate within 3%.
How do you convert °F to °C? Subtract 32 and multiply by 5/9: (°F − 32) × 5/9. For temperature differences only, use Δ°F × 5/9 = Δ°C (no offset). 20°F difference = 11.1°C difference.
Is 1 kg equal to 2.2 lb? In mass terms yes — 1 kg = 2.2046 lb. But weight (force) conversions depend on gravity: 1 kg mass exerts 9.81 N, while 1 lb mass exerts 1 lbf. In engineering stress and pressure calculations, always distinguish mass from force.
Why do pipe sizes not convert exactly? NPS and DN are nominal designations, not exact dimensions: NPS 12" OD is 323.9 mm, not 300 mm (DN300). Schedule (wall thickness) is separately specified. Always use actual OD/WT from the pipe standard, not a unit conversion.
What is the conversion between horsepower and kilowatts? 1 hp (mechanical) = 0.7457 kW; 1 metric hp (PS) = 0.7355 kW. For motor nameplates, US uses hp, international uses kW — see Motor Efficiency Guide.
How many US gallons in a cubic meter? 264.17 US gallons (231 in³ per gallon). An oil barrel (bbl) is 42 US gallons = 0.159 m³.
How do I convert airflow CFM to m³/h? Multiply CFM by 1.699: 10,000 CFM = 16,990 m³/h = 4.72 m³/s. Use the Flow Conversion Calculator for flow units.
How do I convert dynamic viscosity from centipoise to Pa·s? 1 cP = 0.001 Pa·s (1 mPa·s). Water at 20°C is 1.002 cP ≈ 0.001 Pa·s. To convert cP to Pa·s, divide by 1000; to convert Pa·s to cP, multiply by 1000. Kinematic viscosity in cSt converts to m²/s by dividing by 1,000,000 (1 cSt = 1×10⁻⁶ m²/s).
Summary
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.