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.