A water pipe has to carry its flow at a sensible speed — fast enough not to let sediment settle, slow enough not to roar or erode fittings. The standard quick method is the same as for ducts: pick a target velocity, and the required bore falls out of the flow rate. Then round up to the next standard pipe size.
The velocity method
The required area is the flow divided by the target velocity (A = Q ÷ v), and the diameter follows from d = √(4A ÷ π). For example, 8 GPM at 6.5 ft/s (30 L/min at 2 m/s) needs about a 0.7 in (17.8 mm) bore — so the next standard size up is a ¾ in (22 mm) pipe.
What water velocity to use
Design practice keeps water between about 3 and 8 ft/s (0.9–2.4 m/s):
| Run | Velocity (ft/s) | m/s |
|---|---|---|
| Branch to fixture (quiet) | 4–5 | 1.2–1.5 |
| Main / riser | ~6.5 | ~2 |
Above roughly 8 ft/s pipes get noisy and fittings erode — especially copper on hot water — and below about 2 ft/s sediment can settle in long runs.
Standard pipe sizes
NPS sizes run ½″, ¾″, 1″, 1¼″, 1½″ and 2″, matching metric copper 15, 22, 28, 35, 42 and 54 mm (outside diameter). Size on the internal bore — plastic pipes (PEX, CPVC, PVC) have thicker walls, so ¾″ PEX carries less than ¾″ copper. Check the bore for the material you are installing.
Pipe Size Calculator
Enter the flow rate in GPM (L/min) and a target velocity — it returns the internal diameter, the standard NPS size, and the actual velocity through it.
What flow do fixtures need?
Rough per-fixture demand:
| Fixture | Flow (GPM) |
|---|---|
| Bathroom faucet | 1.5–2.2 |
| Shower | 2–2.5 |
| Kitchen faucet | ~2.2 |
| Tub filler | ~4 |
For a pipe feeding several fixtures, don’t just add them all — not everything runs at once. Size for the fixtures that realistically run together, or apply your local code’s fixture-unit method.
When to check pressure drop
Velocity sizing lands the right size for typical residential and light commercial runs, but it doesn’t calculate pressure loss. Long runs, many fittings or low supply pressure need a friction-loss check so the far fixture still gets its flow — and your local plumbing code takes precedence.