Plumbing
GPM Calculator
Convert between flow rate units and check a measured or required flow against a pipe size.
Result
US gallons throughout. Imperial gallons are about 20% larger, so check which a pump or fixture specification refers to.
Get a Free Business AuditWhat moves the number
- Whether the figure is peak or average, which differ by a large factor.
- Simultaneous use, since total fixture capacity is never all demanded at once.
- The measurement method, because a timed fill and a meter reading can disagree.
Where this one goes wrong
Adding up every fixture's rated flow to size a supply. No building ever runs every outlet at once, and sizing on the total produces pipe far larger than the code method would require.
Questions
About this calculator
How do I measure GPM on site?
Time how long a known volume takes to fill. A bucket of known size and a stopwatch gives a figure accurate enough for almost any service decision.
Why not size on total fixture flow?
Because simultaneous demand is far lower than the sum. Codes use fixture unit methods precisely because they model probability of simultaneous use rather than worst-case addition.
What flow does a typical fixture need?
It varies by fixture type and by the water efficiency standards in force, which have changed over time. Use the fixture's own rating rather than a remembered figure.
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How to use this calculator
- Flow value. Enter your own figure; the page starts at 12.
- Unit. Enter your own figure; the page starts at Gallons per minute (US).
The result updates as you type, so you can try a figure, see what it does to gallons per minute, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Flow value | 12 |
| Unit | Gallons per minute (US) |
What comes out
| Result | Value |
|---|---|
| Gallons per minute main result | 12.0 |
| Litres per minute | 45.4 |
| Gallons per hour | 720 |
| Cubic metres per hour | 2.7255 |
That is 12.0 GPM, 45.4 litres per minute, or 720 gallons per hour.
How sensitive the answer is
Holding everything else steady and changing only flow value, this is what happens to gallons per minute. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Flow value | Gallons per minute |
|---|---|
| 6 | 6.0 |
| 9 | 9.0 |
| 12 example above | 12.0 |
| 18 | 18.0 |
| 24 | 24.0 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Flow value
- A number, between 0 and 1000000, starting at 12.
- Unit
- Choose from Gallons per minute (US), Litres per minute, Gallons per hour, Cubic feet per second, Cubic metres per hour.
What the results tell you
| Result | What it is |
|---|---|
| Gallons per minute | The headline result. A decimal figure. |
| Litres per minute | A decimal figure. |
| Gallons per hour | A number. |
| Cubic metres per hour | A number. |
Before you rely on this number
Pipe sizing, flow and pressure figures depend on the actual fittings, the run, the material and local code. Code requirements vary by jurisdiction and override any general calculation, so check what applies where the work is happening.
Keep going
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