Plumbing
Water Flow Calculator
Work out volumetric flow from pipe size and velocity, or check one against the other.
Result
A bucket test measures flow at one fixture with everything else closed. Simultaneous demand elsewhere in the house will reduce it, so test at the worst-case fixture.
Get a Free Business AuditWhat moves the number
- Inside diameter, since flow rises with the square of the radius.
- Velocity, which is limited by erosion and noise rather than by physics.
- Whether the figure needed is peak demand or continuous.
Where this one goes wrong
Designing to the maximum velocity the pipe can survive. It meets the flow requirement on paper and produces a noisy system that wears, and the saving is one pipe size.
Questions
About this calculator
How much more flow does the next pipe size give?
Considerably more than the size step suggests, because area rises with the square of the radius. Going up one nominal size is frequently the cheapest fix for a marginal run.
What is a reasonable design velocity?
Lower than the maximum, and lower again on hot lines where erosion is faster. Design to the comfortable range rather than the limit.
Does this account for fittings?
No. It gives flow for a pipe at a velocity. Friction loss across a real run needs the equivalent length of every fitting on the path.
The formula
GPM = container volume ÷ fill time in seconds × 60
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How to use this calculator
- Container volume (gallons). Enter your own figure; the page starts at 5.
- Time to fill (seconds). Enter your own figure; the page starts at 24.
- Fixtures you want to run at once. Enter your own figure; the page starts at 3.
- Flow needed per fixture (GPM). Enter your own figure; the page starts at 2.5.
The result updates as you type, so you can try a figure, see what it does to measured flow rate, 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 |
|---|---|
| Container volume (gallons) | 5 |
| Time to fill (seconds) | 24 |
| Fixtures you want to run at once | 3 |
| Flow needed per fixture (GPM) | 2.5 |
What comes out
| Result | Value |
|---|---|
| Measured flow rate main result | 12.5 |
| Flow needed for those fixtures | 7.5 |
| Surplus or shortfall | 5.0 |
| Gallons per hour | 750 |
Measured flow is 12.5 GPM, comfortably above the 7.5 GPM those 3 fixtures need.
How sensitive the answer is
Holding everything else steady and changing only container volume (gallons), this is what happens to measured flow rate. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Container volume (gallons) | Measured flow rate |
|---|---|
| 2.5 | 6.3 |
| 3.75 | 9.4 |
| 5 example above | 12.5 |
| 7.5 | 18.8 |
| 10 | 25.0 |
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
- Container volume (gallons)
- A number, between 0 and 500, starting at 5.
- Time to fill (seconds)
- A number, between 0 and 3600, starting at 24.
- Fixtures you want to run at once
- A number, between 0 and 100, starting at 3.
- Flow needed per fixture (GPM)
- A number, between 0 and 50, starting at 2.5.
What the results tell you
| Result | What it is |
|---|---|
| Measured flow rate | The headline result. A decimal figure. |
| Flow needed for those fixtures | A decimal figure. |
| Surplus or shortfall | A decimal figure. |
| Gallons 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.
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