Conversion from US Gallons per minute to Cubic inches per hour

Information about the 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.

Information about the Cubic inch per hour (in³/h)

The cubic inch per hour is a unit of volumetric flow rate equal to one cubic inch passing a point every hour. Its symbol is in³/h. Since a cubic inch is a little over a tablespoon, an hour at this rate produces a small puddle — which is exactly why the unit belongs to slow losses, slow doses and slow wear rather than to transfers.

Seepage and weeping are its natural subject. A mechanical seal on a pump is allowed a small permanent leakage to lubricate its faces, and the specification is written in cubic inches per hour on American equipment. A packed gland on a valve stem is similar: a few drops an hour is correct operation, and a dry gland is a gland about to score its stem.

Oil consumption in engines is quoted this way in the imperial world. A large stationary engine's cylinder lubrication is metered at a few cubic inches per hour per cylinder, and the acceptable rate of oil loss past the rings on a diesel is expressed the same way. Because such engines run for thousands of hours between overhauls, an hourly figure is what turns into a drum of oil on the purchase order.

Slow-fill and top-up systems live here too. An automatic battery-watering system, a coolant make-up line, a header tank feeding a boiler and a chemical dosing pump on a cooling circuit all move volumes of this size, and their design lifetime is set by the reservoir divided by the hourly rate.

Corrosion, erosion and permeation figures convert into it when a rate must be given as a volume. A seal that permits so many cubic inches of refrigerant per hour, or a hose whose wall passes so much fuel vapour, is compared against a regulatory limit written as an annual figure, and the hourly rate is what the test bench actually measures.

The relation to larger units frames it clearly. There are 231 cubic inches in a US gallon, so one cubic inch per hour fills a gallon in about ten days, and 61 cubic inches per hour is roughly a litre per hour. A flow that takes a week and a half to fill a milk jug is the very definition of a rate you watch rather than use.

One cubic inch per hour equals about 16.39 cubic centimetres per hour, about 0.01667 cubic inches per minute, or about 0.004329 US gallons per hour.