Torque, pressure and power: a field guide to engineering conversions
The engineering conversions that trip people up most — torque vs energy, gauge vs absolute pressure, and the three horsepowers.
Engineering drawings, spec sheets and service manuals mix SI and US units freely. These are the conversions where mistakes are most common and most costly.
Torque is not energy
Torque (N·m, lbf·ft) and energy (J, ft·lbf) have the same dimensions, but they are different quantities: torque is a twisting force about an axis, energy is work done. Keep them apart, and watch the word order — “foot-pounds” often means energy while “pound-feet” means torque, though manuals use both loosely.
| Pound-feet (lbf⋅ft) | Newton meters (N⋅m) |
|---|---|
| 10 lbf⋅ft | 13.558179 N⋅m |
| 25 lbf⋅ft | 33.895449 N⋅m |
| 50 lbf⋅ft | 67.790897 N⋅m |
| 80 lbf⋅ft | 108.46544 N⋅m |
| 100 lbf⋅ft | 135.58179 N⋅m |
| 150 lbf⋅ft | 203.37269 N⋅m |
| 200 lbf⋅ft | 271.16359 N⋅m |
Small fasteners are specified in inch-pounds: 1 lbf·in = 0.112985 N·m. Confusing in-lb with ft-lb over-tightens by a factor of 12. See the torque converter.
Gauge vs absolute pressure
A tyre gauge reading 32 psi means 32 psi above atmospheric pressure (psig). The absolute pressure is about 32 + 14.7 = 46.7 psia. Unit conversion does not change the reference: 32 psig is 2.206 bar gauge, not absolute. Thermodynamic equations (gas laws, steam tables) need absolute pressure.
| Pounds per square inch (psi) | Bars (bar) |
|---|---|
| 15 psi | 1.0342136 bar |
| 30 psi | 2.0684272 bar |
| 32 psi | 2.2063223 bar |
| 35 psi | 2.4131651 bar |
| 50 psi | 3.4473786 bar |
| 100 psi | 6.8947573 bar |
| 150 psi | 10.342136 bar |
| 3,000 psi | 206.84272 bar |
Three different horsepowers
- Mechanical (imperial) horsepower: 550 ft·lbf/s = 745.699872 W — US engine ratings.
- Metric horsepower (PS, CV, pk): 735.49875 W exactly — European brochures.
- Electrical horsepower: 746 W exactly — motor nameplates.
A “300 PS” engine is 295.9 mechanical hp and 220.65 kW.
Flow rates
Pumps are rated in gallons per minute in the US and m³/h or L/min elsewhere; fans in CFM. 1 GPM = 3.785411784 L/min and 1 CFM = 1.69901 m³/h. Remember that volumetric flow of a gas depends on pressure and temperature — for gases, “SCFM” is referenced to standard conditions, which vary between standards.
Stress and modulus
Material strengths are given in MPa (= N/mm²) or ksi. 1 ksi = 6.894757293 MPa, so structural steel with 250 MPa yield strength is about 36.26 ksi.