Plumbing
Plumbing Pipe Size Calculator
Check supply pipe size against demand, pressure available and the length the water has to travel.
Result
Pipe sizing, venting and drainage are set by the plumbing code adopted where you are, and the IPC and UPC do not agree with each other. Treat this as a planning figure and size the finished design from the code table your inspector works to. Velocity sizing ignores friction loss along the run and through fittings, which on a long or convoluted run is what actually limits flow. Hot water is held to a lower velocity because erosion corrosion attacks copper faster at temperature.
Get a Free Business AuditWhat moves the number
- Static pressure at the meter and how much of it the run consumes.
- Total developed length, where fittings often contribute more than the straight pipe.
- Simultaneous demand rather than total fixture count.
Where this one goes wrong
Sizing the main correctly and ignoring the branches. A correctly sized trunk feeding undersized branches still produces poor flow at the fixture, which is what the customer actually experiences.
Questions
About this calculator
Why does my flow drop when two fixtures run?
Because the pipe is sized for one. Simultaneous demand is what sizing has to cover, and a system that performs well with a single outlet open can fail entirely with two.
How much does elevation cost in pressure?
Roughly 0.433 psi for every foot of rise. On a multi-storey building that is a substantial share of the available pressure before any friction loss is counted.
Do fittings really matter that much?
On a run with many elbows and tees, yes. Their equivalent length can exceed the straight pipe, and a calculation that counts only the pipe comes out undersized.
The formula
pipe area = GPM / (448.8 x velocity in ft/s); diameter = sqrt(4 x area / pi)
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How to use this calculator
- Flow required (GPM). Enter your own figure; the page starts at 12.
- Pipe run. Enter your own figure; the page starts at Branch, cold (8 ft/s).
- Material. Enter your own figure; the page starts at Copper type L.
The result updates as you type, so you can try a figure, see what it does to internal diameter needed, 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 required (GPM) | 12 |
| Pipe run | Branch, cold (8 ft/s) |
| Material | Copper type L |
What comes out
| Result | Value |
|---|---|
| Internal diameter needed main result | 0.8 |
| Nominal pipe size | 3/4 |
| Design velocity | 8.0 |
| Velocity in the chosen size | 8.0 |
12 GPM at 8.0 ft/s needs 0.8 inches of bore, so use 3/4 inch pipe, which runs at 8.0 ft/s.
How sensitive the answer is
Holding everything else steady and changing only flow required (gpm), this is what happens to internal diameter needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Flow required (GPM) | Internal diameter needed |
|---|---|
| 6 | 0.6 |
| 9 | 0.7 |
| 12 example above | 0.8 |
| 18 | 1.0 |
| 24 | 1.1 |
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
- Flow required (GPM)
- A number, between 0 and 2000, starting at 12.
- Pipe run
- Choose from Service main (8 ft/s), Branch, cold (8 ft/s), Hot water (5 ft/s, erosion limit), Keep it quiet (4 ft/s).
- Material
- Choose from Copper type L, PEX, CPVC.
What the results tell you
| Result | What it is |
|---|---|
| Internal diameter needed | The headline result. A decimal figure. |
| Nominal pipe size | A category or planning selection, rather than a measured quantity. |
| Design velocity | A decimal figure. |
| Velocity in the chosen size | 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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