What this calculator does
GPM stands for gallons per minute, the standard way flow rate is expressed for pumps, taps, hoses, sprinkler heads and irrigation systems. The GPM formula is simple: take the volume of liquid that passed a point, in gallons, and divide it by the time that took, in minutes.
The easiest way to measure flow rate in practice is to time how long a container of known size takes to fill, then let this calculator do the division. It also converts the result to litres per minute, since plumbing fixtures and pump specifications are not always labelled in the same unit.
The formula
Divide the volume collected, in gallons, by the time taken to collect it, in minutes. Choosing US or imperial gallon only changes the litres-per-minute conversion shown alongside the GPM figure, since GPM itself is just volume over time in whichever gallon you measured with.
| Term | Meaning |
|---|---|
| GPM | Gallons per minute: volume in gallons divided by time in minutes. |
| Volume | The amount of liquid collected or moved during the timed period. |
| Time | How long it took for that volume to pass, in minutes. |
The inputs explained
| Field | What to enter |
|---|---|
| Volume collected (gal) | The volume of liquid you measured, such as a bucket you timed filling up. |
| Time taken (min) | How long it took to collect that volume, in minutes (0.5 for 30 seconds). |
| Gallon type | US and imperial gallons differ in size; pick whichever the volume was measured in. |
When to use it
Testing a tap or shower head
Time how long a bucket of known volume takes to fill, then convert that into a GPM figure to compare against a fixture's rated flow or a local water-efficiency limit.
Sizing a pump or irrigation line
Pump and sprinkler specifications are usually quoted in GPM, so a field measurement of volume and time needs converting to the same unit before it can be compared against the rated capacity.
Checking a well or bore's yield
Timing how long a well takes to deliver a known volume gives a rough GPM yield figure, useful before sizing a pump or storage tank for that supply.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What GPM results from different fill times, for a fixed volume?
The same 5 gallon container, timed at a range of fill speeds.
| Time taken | Flow rate | Flow rate in litres per minute |
|---|---|---|
| 0.25 min | 20.00 GPM | 75.71 L/min |
| 0.5 min | 10.00 GPM | 37.85 L/min |
| 1 min | 5.00 GPM | 18.93 L/min |
| 2 min | 2.50 GPM | 9.46 L/min |
| 3 min | 1.67 GPM | 6.31 L/min |
| 5 min | 1.00 GPM | 3.79 L/min |
How does GPM change with the volume collected in a fixed one-minute test?
A one-minute timed collection, at a range of volumes.
| Volume collected | Flow rate | Flow rate in litres per minute |
|---|---|---|
| 1 gal | 1.00 GPM | 3.79 L/min |
| 2 gal | 2.00 GPM | 7.57 L/min |
| 5 gal | 5.00 GPM | 18.93 L/min |
| 10 gal | 10.00 GPM | 37.85 L/min |
| 15 gal | 15.00 GPM | 56.78 L/min |
| 20 gal | 20.00 GPM | 75.71 L/min |
Questions
What is the GPM formula?
GPM = volume in gallons divided by time in minutes. If a container holds 5 gallons and takes 30 seconds (0.5 minutes) to fill, the flow rate is 5 divided by 0.5, which is 10 GPM.
What is GPM used for?
It is the standard unit for describing how fast a pump, tap, shower head, hose or irrigation line moves water, and is used to size pumps, check fixtures against water-efficiency limits, and estimate well or bore yield.
Does US or imperial gallon matter for the GPM figure?
Only for the litres-per-minute conversion shown alongside it. The GPM number itself is just the volume you measured divided by time, in whichever gallon size that volume was recorded in.
How do I convert an existing GPM figure to litres per minute?
If you already have a GPM number rather than a volume and time to measure, the GPM to LPM calculator converts it directly, in either direction.
Already have a flow rate in GPM and just need it in litres per minute or back again, see the GPM to LPM calculator.