Gallons Per Minute Calculator (GPM)

The simplest way is a timed fill test. Put a container of known size under the fixture, open it fully, and time how long it takes to fill. Then divide the gallons collected by the seconds it took and multiply by 60.

For example, if a 5-gallon bucket fills in 40 seconds, that is 5 divided by 40 times 60, which is 7.5 gallons per minute.

Run the test a few times and average the results for a reliable figure. If you measured in liters or quarts, convert to gallons first — 1 liter is 0.264 gallon and 1 quart is 0.25 gallon — and if you timed in minutes, use that directly instead of multiplying by 60.

For a whole house, a flow of roughly 6 to 12 gallons per minute at the main is generally comfortable, letting you run a couple of fixtures at once without a noticeable pressure drop. Below about 3 GPM feels weak, and above 12 is strong.

At the fixture, flow rates are lower and often capped by code: modern shower heads are limited to about 2.5 GPM, bathroom faucets to around 1.5 GPM, and kitchen faucets to about 2.2 GPM.

What counts as adequate depends on how many fixtures run together and your household’s habits. This calculator rates your measured flow and shows how many common fixtures it could supply at once.

A residential well is often considered adequate at around 5 gallons per minute of sustained yield, though many lenders and health departments look for a minimum closer to 3 to 5 GPM. A strong well may produce 10 GPM or more.

The important figure is the sustained yield, not a brief burst. A quick bucket test measures the flow coming out of the pressure tank, which can read high because the tank is simply emptying; the well’s actual recovery rate may be lower once the tank draws down.

For well-yield decisions — sizing a pump, buying a property, or troubleshooting low water — have a licensed well driller run a proper draw-down or well-yield test rather than relying on a short fixture test.

To go from gallons per minute to gallons per hour, multiply by 60, since there are 60 minutes in an hour. To get gallons per day, multiply the GPM figure by 1,440, the number of minutes in a day.

So a flow of 5 GPM is 300 gallons per hour and 7,200 gallons per day if it ran continuously.

These larger figures are useful for sizing storage tanks, estimating irrigation volumes, or working out how long a continuous fill would take. This calculator shows GPH and GPD automatically, along with metric rates like liters per minute and cubic meters per hour.

Divide the tank’s capacity in gallons by the flow rate in gallons per minute. The result is the fill time in minutes.

For example, filling a 300-gallon tank at 6 GPM takes 300 divided by 6, which is 50 minutes. A 1,000-gallon pool-top-up at 10 GPM takes 100 minutes, or about 1 hour 40 minutes.

Keep in mind that flow can fall as a tank or pressure system draws down, so a real fill may take a little longer than the simple calculation suggests. This calculator works out the fill time for any tank, pool or container size you enter.

On a well system, a short bucket test often reads higher than the well’s true sustained output. That is because the water is coming from the pressure tank, which stores a reserve and empties quickly, so for the first minute or two you are measuring the tank emptying, not the well producing.

As the tank draws down and the pump has to keep up on its own, the flow settles to the well’s actual recovery rate, which can be noticeably lower.

To gauge sustained yield, run the water for a longer period — many minutes — and measure the flow once it stabilizes, or have a driller perform a proper well-yield test. A short test is fine for a fixture’s flow, but not for judging a well.

GPM (gallons per minute) measures flow — how much water moves past a point in a given time. PSI (pounds per square inch) measures pressure — the force pushing the water. They are related but not the same.

A system can have high pressure but low flow if the pipe or fixture is restrictive, or good flow at modest pressure through a wide, open line. Pressure drives flow, but the pipe size, fixtures, and restrictions determine how much flow a given pressure actually delivers.

This calculator measures flow (GPM) from a timed fill. To measure pressure you need a gauge on a hose bib or the main; most homes sit around 40 to 60 PSI. Both matter for how a system performs.

A bucket-and-stopwatch test is accurate enough for most practical purposes, and it is how plumbers often check a fixture’s flow. The main sources of error are timing the start and stop, splashing or overfilling, and using a container whose true volume differs from its label.

To improve accuracy, use a larger container so the fill takes longer and timing errors matter less, start timing the instant water hits the container, and run the test three times and average. Open the fixture fully each time.

For a fixture or hose, this is reliable. For a well’s sustained yield or for pump sizing, a short test can mislead — use a longer draw-down test or a professional measurement instead.