Plumbing
Pipe Size Calculator
Size supply pipe from demand and the velocity the material will tolerate, as a planning check against the code that applies where you work.
Result
Pipe and drain sizing is governed by plumbing code (IPC/UPC or local equivalent) and must be designed and installed by a licensed plumber. This is an educational estimate, not a design. Nominal pipe sizes differ from true internal diameter, which varies by material and schedule. This uses nominal size as an approximation.
Get a Free Business AuditWhat moves the number
- Fixture demand, which is not the sum of every fixture running at once.
- Velocity limits, since copper erodes above a certain flow and noise rises with it.
- Pipe material, because copper, PEX and CPVC have different inside diameters at the same nominal size.
- Developed length including fittings, which is what the pressure has to cover.
Where this one goes wrong
Sizing on nominal diameter across materials. PEX has a smaller inside diameter than copper at the same nominal size, so a like-for-like swap by label can undersize the run noticeably.
Before you quote
- The fixture unit method the local code actually requires.
- Available static pressure at the meter, measured rather than assumed.
- Whether a pressure reducing valve is limiting what reaches the building.
Questions
About this calculator
Does this replace a code sizing calculation?
No. Supply sizing is governed by the plumbing code in force where you work, and the fixture unit method and allowable velocities differ between codes. Use this to sanity check a design, not to submit one.
Why does velocity matter as well as flow?
Because high velocity erodes copper over time and makes the system noisy. Meeting the flow requirement with a pipe that is too small trades a long-term failure for a short-term saving.
Does PEX size the same as copper?
Not at the same nominal label. PEX has thicker walls and a smaller bore, so an equivalent run often needs to go up a size to deliver the same flow.
The formula
GPM = 2.448 × diameter² × velocity (diameter in inches, velocity in ft/s)
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How to use this calculator
- Required flow (GPM). Enter your own figure; the page starts at 12.
- Maximum velocity (ft/s). Codes commonly cap cold water around 8 ft/s and hot water around 5.
The result updates as you type, so you can try a figure, see what it does to recommended pipe size, 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 |
|---|---|
| Required flow (GPM) | 12 |
| Maximum velocity (ft/s) | 6 |
What comes out
| Result | Value |
|---|---|
| Recommended pipe size main result | 1" |
| Minimum internal diameter | 0.9 |
| Velocity at that size | 4.9 |
| Capacity of the recommended size | 14.7 |
To carry 12 GPM below 6 ft/s you need at least 0.9" internal diameter. Use 1", which runs at 4.9 ft/s.
How sensitive the answer is
Holding everything else steady and changing only required flow (gpm), this is what happens to recommended pipe size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Required flow (GPM) | Recommended pipe size |
|---|---|
| 6 | 3/4" |
| 9 | 1" |
| 12 example above | 1" |
| 18 | 1-1/4" |
| 24 | 1-1/2" |
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
- Required flow (GPM)
- A number, between 0 and 5000, starting at 12.
- Maximum velocity (ft/s)
- Codes commonly cap cold water around 8 ft/s and hot water around 5.
What the results tell you
| Result | What it is |
|---|---|
| Recommended pipe size | The headline result. A category or planning selection, rather than a measured quantity. |
| Minimum internal diameter | A decimal figure. |
| Velocity at that size | A decimal figure. |
| Capacity of the recommended 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.
Keep going
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