Plumbing
Pipe Flow Calculator
Work out flow and friction loss for a pipe run, so the size can be checked against both delivery and pressure.
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. Friction loss uses a Hazen-Williams coefficient of 150, typical for smooth plastic pipe. Older galvanised or scaled pipe loses far more, and fittings add equivalent length this does not count.
Get a Free Business AuditWhat moves the number
- Inside diameter rather than nominal size, which differ by material.
- Pipe roughness, since the friction coefficient varies by material and age.
- Length including the equivalent length of every fitting.
Where this one goes wrong
Using a clean pipe coefficient on old galvanised. Decades of scale reduce the effective bore substantially, and a calculation on the nominal size will not match what the building actually delivers.
Questions
About this calculator
Why is my old system slower than the calculation says?
Usually scale. Galvanised pipe narrows internally over decades, so the effective diameter is smaller than the label and the friction loss is far higher than a clean pipe figure.
What velocity is too high?
It depends on the material and on whether the line is hot, because erosion accelerates with temperature. Codes and manufacturers publish limits and those are the figures to design against.
Does this size a pipe?
It tells you what a given pipe delivers and loses. Sizing for a building is a code calculation with fixture units and simultaneous demand, which is a different exercise.
The formula
GPM = 2.448 × d² × velocity; friction loss estimated by Hazen-Williams (C = 150)
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How to use this calculator
- Pipe size. Enter your own figure; the page starts at 3/4".
- Water velocity (ft/s). Enter your own figure; the page starts at 5.
- Pipe run length (ft). Enter your own figure; the page starts at 60.
The result updates as you type, so you can try a figure, see what it does to 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 |
|---|---|
| Pipe size | 3/4" |
| Water velocity (ft/s) | 5 |
| Pipe run length (ft) | 60 |
What comes out
| Result | Value |
|---|---|
| Flow rate main result | 6.9 |
| Gallons per hour | 413 |
| Cross-sectional area | 0.441786 |
| Approx. friction loss over the run | 3.7 |
A 3/4" pipe at 5 ft/s carries 6.9 GPM, losing about 3.7 PSI over 60 feet.
How sensitive the answer is
Holding everything else steady and changing only water velocity (ft/s), this is what happens to flow rate. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Water velocity (ft/s) | Flow rate |
|---|---|
| 2.5 | 3.4 |
| 3.75 | 5.2 |
| 5 example above | 6.9 |
| 7.5 | 10.3 |
| 10 | 13.8 |
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
- Pipe size
- Choose from 1/2", 3/4", 1", 1-1/4", 1-1/2", 2", 3", 4", 6".
- Water velocity (ft/s)
- A number, between 0 and 30, starting at 5.
- Pipe run length (ft)
- A number, between 0 and 10000, starting at 60.
What the results tell you
| Result | What it is |
|---|---|
| Flow rate | The headline result. A decimal figure. |
| Gallons per hour | A number. |
| Cross-sectional area | A number. |
| Approx. friction loss over the run | 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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