Calculators
Pipe Size and Flow Rate Calculator
Calculate water velocity and friction pressure loss in copper pipe sizes for a flow rate, and the smallest size within a velocity limit.
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- Nothing uploaded
The tool
Enter your numbers below. Results update as you type, and nothing leaves your browser.
Everything you enter stays on your own machine. Nothing is sent to us or to anyone else, which you can confirm in your browser network tab.
What people use it for
- Sizing a water main or branch
- Checking pressure at the far end of a run
- Designing recirculation loops
- Irrigation and equipment connections
About this tool
What the Pipe Size and Flow Rate Calculator does
Pipe size is a balance between velocity and pressure loss. Water moving too fast causes noise, erosion and pressure loss, so designers cap velocity, commonly around 8 feet per second for cold water and 5 for hot. This calculator works out velocity and Hazen–Williams friction loss for standard copper sizes.
It also works as a pipe size flow rate calculator or a water pipe sizing calculator.
Formula
- Velocity (ft/s) = 0.4085 × GPM ÷ inside diameter²
- Head loss (ft per 100 ft) = 0.2083 × (100 ÷ C)^1.852 × GPM^1.852 ÷ d^4.8655
- Pressure loss (psi) = head loss × 0.433
Worked example
Using the starting values in the calculator above:
| Input | Value |
|---|---|
| Flow rate | 10 gal/min |
| Maximum velocity | 8 ft/s |
| Hazen–Williams C factor | 140 |
| Pipe length | 100 ft |
| Result | Value |
|---|---|
| Smallest pipe within the velocity limit | 3/4 in |
| Velocity in that pipe | 6.63 |
| Friction loss over the length | 11.17 |
How to read the result
Fittings add equivalent length; add 25% to 50% to the straight length for a typical run with bends and valves.
Plumbing codes size fixture supplies with fixture units; use this for mains, recirculation and equipment connections.
For planning only. Electrical, gas and refrigerant work must follow current codes and be carried out or checked by a licensed professional.
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Questions
Questions about this tool
The things clients actually want to know — costs, timelines, ownership and what happens after launch.
Commonly about 8 ft/s for cold water and 4 to 5 ft/s for hot water to limit noise and erosion.
An empirical formula for friction loss in water pipes based on flow, diameter and a roughness factor C.
Yes, standard inside diameters for copper Type L tube.
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