| Cubic feet per hour (ft³/h) | Gallons per minute (gal/min) |
|---|---|
| 1 Cubic foot per hour | 0.10381392431 gal/min |
| 2 Cubic feet per hour | 0.20762784862 gal/min |
| 3 Cubic feet per hour | 0.31144177293 gal/min |
| 4 Cubic feet per hour | 0.41525569724 gal/min |
| 5 Cubic feet per hour | 0.51906962155 gal/min |
| 10 Cubic feet per hour | 1.0381392431 gal/min |
| 20 Cubic feet per hour | 2.0762784862 gal/min |
| 25 Cubic feet per hour | 2.59534810775 gal/min |
| 50 Cubic feet per hour | 5.1906962155 gal/min |
| 100 Cubic feet per hour | 10.381392431 gal/min |
| Reference | Cubic feet per hour (ft³/h) | Gallons per minute (gal/min) |
|---|---|---|
| A domestic shower | 19.0699 ft³/h | 1.97972 gal/min |
| A kitchen tap | 21.1888 ft³/h | 2.19969 gal/min |
| A garden hose | 31.7832 ft³/h | 3.29954 gal/min |
| The Amazon river | 2.65708 × 1010 ft³/h | 2.75841 × 109 gal/min |
The cubic foot per hour is a unit of volumetric flow rate equal to one cubic foot passing a point every hour. Its symbol is ft³/h, and in the gas trades it is abbreviated CFH. It is the unit of fuel gas supply throughout North America, where every appliance and every pipe is sized by it.
Natural gas is billed and metered in cubic feet, so the hourly rate is what a gas fitter works with. A domestic furnace consumes 80 to 150 cubic feet per hour, a water heater 40, a cooking range 65, and a whole house at design conditions perhaps 250. A residential meter is rated at 250 CFH and the service pipe at rather more.
Pipe sizing tables are built entirely around this unit. Given a length of pipe, a diameter and an allowable pressure drop, the table returns a capacity in cubic feet per hour, and the fitter compares it against the sum of the appliance ratings on that branch. Undersize the pipe and the burners starve when everything runs at once; oversize it and the material cost rises for no benefit.
The unit converts easily into heat. Natural gas carries roughly 1,000 British thermal units per cubic foot, so a rate in cubic feet per hour is very nearly the same number in thousands of British thermal units per hour. An appliance rated 100,000 British thermal units per hour therefore burns about 100 cubic feet per hour, which is why the two figures appear together on every rating plate.
Propane behaves differently and is worth stating separately. It carries about 2,500 British thermal units per cubic foot, so the same appliance needs less than half the volumetric flow, and a pipe sized for natural gas is generously large for propane. Confusing the two is a classic and dangerous error in the trade.
Beyond fuel gas, the unit describes slow air and gas flows in imperial practice: purge lines, instrument air, small blowers and laboratory gas supplies. Sixty cubic feet per hour is one cubic foot per minute, and 35.31 cubic feet per hour is one cubic metre per hour.
One cubic foot per hour equals about 0.02832 cubic metres per hour, about 28.32 litres per hour, or about 0.01667 cubic feet per minute.
The gallon per minute is a unit of volumetric flow rate equal to one imperial gallon passing a point every minute. Its symbol is gal/min, usually abbreviated GPM. The imperial gallon is 4.54609 litres, so this unit sits comfortably in the range of pumps, wells and building services, which is where it did most of its work before metrication.
Water wells and boreholes are still described by it in Britain and Ireland. A domestic borehole yields 2 to 10 gallons per minute, an agricultural one 20 to 60, and a public supply borehole several hundred. The yield test that establishes the figure runs the pump for hours while watching the water level, and the sustainable yield in gallons per minute is what determines whether a licence can be granted.
Pumps and pumping stations were rated in it throughout the industrial era. Boiler feed pumps, sewage pumps and fire pumps all carried plates in gallons per minute, and older British pump curves plot head against this unit rather than against cubic metres per hour. Anyone matching a replacement pump to an old installation has to convert the duty point carefully.
Building services used it for hot and cold water services, heating circuits and cooling towers. A heating circuit's flow follows from the heat it must carry and the temperature drop across the circuit, and in imperial terms the arithmetic came out in gallons per minute against a drop in degrees Fahrenheit — a combination that survives in many older plant rooms.
The unit also has a firefighting history. British fire brigade pumps were classified as light, medium and heavy by their output in gallons per minute, and hydrant flow tests, hose reel ratings and sprinkler system design all rested on the same figure before European standards moved the trade to litres per minute.
For scale, one gallon per minute is about 4.5 litres per minute, and 13.2 gallons per minute is one litre per second. A domestic shower running at 9 litres per minute is just under 2 gallons per minute, which puts everyday plumbing at the low end of the unit's range.
One gallon per minute equals about 4.546 litres per minute, about 0.2728 cubic metres per hour, or about 1.201 US gallons per minute.