SI vs Imperial: The Unit Traps That Sink Thermodynamics Problems

Most thermodynamics mistakes aren’t conceptual — they’re unit slips. Here are the conversions that matter, and the traps they set.

The core conversions

Quantity	SI	English	Factor
Pressure	1 kPa	0.14504 psi	×0.14504
Temperature	1 °C	1.8 °F	×1.8 + 32
Energy	1 kJ	0.94782 Btu	×0.94782
Specific energy	1 kJ/kg	0.42992 Btu/lbm	×0.42992
Specific volume	1 m³/kg	16.0185 ft³/lbm	×16.0185
Power	1 kW	3.4121 Btu/s	×3.4121

Temperature is the sneaky one: a difference of 1 °C is 1.8 °F, but an absolute temperature converts as °F = 1.8·°C + 32.

Trap 1: the kinetic-energy factor of 1000

In the steady-flow energy equation, h is in kJ/kg and V²/2 is in J/kg. Mixing them silently underestimates velocity by √1000 = 31.6×.

V = sqrt( 2 · Δh[kJ/kg] · 1000 )   ← the 1000 is not optional

In English units the same term needs g_c = 32.174 lbm·ft/(lbf·s²) and a Btu→ft·lbf factor of 778.17. The unit bookkeeping is brutal by hand — which is exactly why the nozzle calculator does it for you.

Trap 2: gauge vs absolute pressure

Property tables use absolute pressure. A problem that says “100 psi gauge” means 100 + 14.7 = 114.7 psia. Plug in the gauge value and you’re in the wrong region of the steam table.

Trap 3: quality is dimensionless

x has no units and no conversion. It’s a mass fraction. Don’t multiply it by anything.

Trap 4: per-mass vs per-mole

The air tables here are per mass (kJ/kg). Some chemistry tables are per mole (kJ/kmol). Divide by molar mass (air ≈ 28.97 kg/kmol) to convert.

The safe path

Pick one unit system, write the conversions at the top of your page, and convert once at the boundary — not mid-equation. The calculators on this site store your inputs in the units you type and convert internally at solve time, so you can work a problem in psi and °F end to end and read the answer back in the same units. Toggle SI/English at the top of any calculator and the labels follow.