How to convert flow rate units
Volumetric flow is volume per time, so converting means swapping the volume unit (gallon, cubic foot, litre, cubic metre) and the time unit (minute, hour, second). Against GPM:
| 1 unit | In GPM | In m³/h |
|---|---|---|
| 1 US GPM | 1 | 0.227125 |
| 1 m³/h | 4.40287 | 1 |
| 1 L/min | 0.264172 | 0.06 |
| 1 L/s | 15.8503 | 3.6 |
| 1 CFM | 7.48052 | 1.69901 |
Definitions used: 1 US gallon = 3.785411784 L, 1 cubic foot = 28.316846592 L and 1 m³ = 1,000 L, all exact.
Worked example
The calculator starts at 50 GPM, a small process pump:
- In m³/h: 50 × 0.227125 = 11.3562 m³/h
- In L/min: 50 × 3.78541 = 189.271 L/min
- In L/s: 3.15451 L/s
- In CFM: 6.68403 CFM
The chart above lists common values from GPM to m³/h and L/min, m³/h to GPM and CFM to m³/h.
Mass flow vs volumetric flow
For liquids, volume barely changes with pressure, so volumetric flow is enough. For gases and compressed air, the same volume weighs more as pressure goes up. That is why compressors rate flow at reference conditions (free air at the inlet, or standard conditions, SCFM). When comparing compressors, check which conditions each maker uses.
Common mistakes
- US vs imperial gallons. Imperial is about 20% bigger.
- Comparing CFM with SCFM without correcting for pressure and temperature.
- Mixing up per minute and per hour. That is a 60 times error.
Pump flow goes hand in hand with pressure: convert psi, bar and kPa with the pressure converter. For bolt torque, use the torque converter.