Plumbing
Plumbing Water Flow Calculator
Check delivered flow at a fixture against what the supply and the pipe run can actually provide.
Result
This is the pipe's capacity at that velocity, not what a system will actually deliver. Available pressure, friction along the run, fittings, valves and the fixture itself all reduce it.
Get a Free Business AuditWhat moves the number
- Pressure at the point of use after elevation and friction are taken out.
- Fixture flow restrictors, which cap delivery regardless of supply.
- Whether the complaint is flow or pressure, which are different problems.
Where this one goes wrong
Treating a flow complaint as a pressure problem. A partially closed valve, a clogged aerator or a restricted branch limits flow while static pressure reads perfectly normal at the gauge.
Questions
About this calculator
What is the difference between flow and pressure?
Pressure is force, flow is volume over time. A system can show good static pressure and poor flow if something downstream is restricting it, which is why both are measured.
Why is only one fixture slow?
Because the restriction is local: an aerator, a supply stop not fully open, a failed cartridge or a kinked flexible connector. Whole-house causes show up everywhere.
How do I measure flow on site?
A bucket and a stopwatch gives a usable figure in seconds and is more reliable than judging by eye. Measured before and after is also the evidence a repair worked.
The formula
GPM = pipe area in square feet x velocity in ft/s x 448.8
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How to use this calculator
- Nominal pipe size. Enter your own figure; the page starts at 3/4 inch.
- Velocity (ft/s). Enter your own figure; the page starts at 6.
The result updates as you type, so you can try a figure, see what it does to flow, 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 |
|---|---|
| Nominal pipe size | 3/4 inch |
| Velocity (ft/s) | 6 |
What comes out
| Result | Value |
|---|---|
| Flow main result | 9.1 |
| Flow | 34.3 |
| Per hour | 543 |
| Internal diameter | 0.8 |
3/4 inch pipe at 6 ft/s carries 9.1 GPM, which is 543 gallons an hour.
How sensitive the answer is
Holding everything else steady and changing only velocity (ft/s), this is what happens to flow. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Velocity (ft/s) | Flow |
|---|---|
| 3 | 4.5 |
| 4.5 | 6.8 |
| 6 example above | 9.1 |
| 9 | 13.6 |
| 12 | 18.1 |
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
- Nominal pipe size
- Choose from 1/2 inch, 3/4 inch, 1 inch, 1-1/4 inch, 1-1/2 inch, 2 inch, 3 inch, 4 inch.
- Velocity (ft/s)
- A number, between 1 and 20, starting at 6.
What the results tell you
| Result | What it is |
|---|---|
| Flow | The headline result. A decimal figure. |
| Flow | A decimal figure. |
| Per hour | A number. |
| Internal diameter | A decimal figure. |
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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