US gallon per minute
Volumetric flow rateexact by definition
The US gallon per minute is the customary volumetric flow unit of North American plumbing, fire protection and pump specification. It equals exactly 6.30901964e-5 cubic metres per second, because the US liquid gallon is defined as exactly 231 cubic inches, which is exactly 3.785411784 litres. The conversion is therefore exact rather than rounded.
Watch out: The imperial gallon used in the United Kingdom and historically in Canada is 4.54609 litres, about 20 percent larger than the US gallon. An imperial gpm is 1.2 US gpm, so a pump curve or a meter from the wrong side of that divide misstates flow by a fifth. Check which gallon before trusting any gpm figure whose origin you do not know.
| 1 gpm | 3.7854118 L/min |
The gallon per minute became the trade convention in North America alongside the horsepower and the foot of head, and every domestic pump curve, fire sprinkler calculation and cooling tower rating in the region is still drawn in it.
About the us gallon per minute
Gallons per minute is the unit almost every North American water system is specified in. A bathroom faucet is limited to 1.2 to 2.2 gpm by code, a shower head to 1.8 to 2.5 gpm, a residential well pump delivers 5 to 15 gpm, and a fire sprinkler head flows perhaps 20 to 30 gpm at its design pressure. The handy metric conversions are that 1 gpm is 0.0631 L/s and about 0.227 m³/h, and that 1 L/s is very nearly 16 gpm.
Two arithmetic shortcuts explain why the unit sticks. Hydraulic power in a pump follows \(P_{\text{hp}} = Q \times H / (3960 \times \eta)\) with \(Q\) in gpm and head \(H\) in feet, where 3960 is the conversion constant that makes the units come out in horsepower. And heat carried by water follows \(q = 500 \times Q \times \Delta T\), giving BTU per hour from gpm and a temperature rise in degrees Fahrenheit, the single most used formula in North American hydronic design. Both constants bake in the gallon, which is why converting a system to metric means relearning the shortcuts rather than just the numbers.