US Gallon per minute (gal US/min)

The US gallon per minute is a unit of volumetric flow rate equal to one United States gallon passing a point every minute. Its symbol is gal US/min, and it is universally abbreviated GPM. It is the single most widely used flow unit in North America, appearing on pumps, wells, plumbing fixtures, irrigation designs and fire codes alike.

Plumbing fixtures are rated in it by federal standard. A showerhead in the United States is limited to 2.5 gallons per minute, a kitchen faucet to 2.2 and a lavatory faucet to 1.5, with stricter figures in some states. Because the flow rate multiplied by the minutes of use gives both the water and the energy to heat it, these limits do more for household energy than their modest numbers suggest.

Pumps are sold on curves drawn against this unit. A shallow-well jet pump delivers 5 to 15 gallons per minute, a submersible well pump 10 to 30, a swimming-pool pump 40 to 80, and a municipal pump thousands. Reading a curve at the required flow gives the head available and the horsepower drawn, which is how a pump is matched to a system.

Irrigation design is built on it. A sprinkler head is rated in gallons per minute, a zone is the sum of its heads, and the zone must not exceed what the supply can deliver — which is itself a figure in gallons per minute measured at the point of connection. Splitting a system into zones is entirely an exercise in staying within that number.

Fire protection uses it as its principal unit. Hydrant flow tests, standpipe requirements, sprinkler design densities and pumper truck ratings are all stated in gallons per minute, from a 1.5-inch hose line at 100 to an engine's 1,500 or more, and the national fire codes are written throughout in these terms.

For scale, one US gallon per minute is about 3.79 litres per minute, and 4.4 gallons per minute is about a cubic metre per hour. The imperial gallon is about 20 per cent larger, so a British pump described as 10 gallons per minute delivers about 12 US gallons per minute — a distinction worth checking on any imported equipment.

One US gallon per minute equals about 3.785 litres per minute, about 0.2271 cubic metres per hour, or about 0.8327 imperial gallons per minute.

US Gallon per minute conversion table