Pipe Size Calculator

It works out how big a water pipe needs to be to carry a given flow at a sensible speed. Enter the flow rate in GPM, pick a target velocity for the type of run, and it returns the internal diameter required, the standard NPS pipe size that covers it, and the actual velocity through that pipe in ft/s.

By the velocity method: the required cross-sectional area is the flow divided by the target velocity (A = Q ÷ v), and the diameter follows from d = √(4A ÷ π). So 8 GPM (30 L/min) at 6.5 ft/s (2 m/s) needs about a 0.7″ (17.8 mm) bore — the next standard size up is a ¾″ (22 mm) pipe.

Design practice keeps water between about 3 and 8 ft/s (0.9–2.4 m/s). Around 6.5 ft/s (2 m/s) suits mains and risers; 4–5 ft/s (1.2–1.5 m/s) is kinder for branches to fixtures, where noise matters. Above roughly 8 ft/s (2.4 m/s) pipes get noisy and fittings erode — especially copper on hot water — and below about 2 ft/s (0.6 m/s) sediment can settle in long runs.

Standard NPS sizes run ½″, ¾″, 1″, 1¼″, 1½″ and 2″, matching the metric copper 15, 22, 28, 35, 42 and 54 mm (outside diameter). The calculator sizes on the internal bore — note that plastic pipes (PEX, CPVC, PVC) have thicker walls, so a ¾″ PEX pipe carries less than ¾″ copper. Check the bore for the material you’re installing.

Roughly: a bathroom faucet 1.5–2.2 GPM, a shower 2–2.5 GPM, a kitchen faucet about 2.2 GPM, a tub filler around 4 GPM, and a toilet refill about 1.5 GPM. 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 simultaneous-demand (fixture-unit) method.

Velocity sizing is the standard quick check and lands the right size for typical residential and light commercial runs. It doesn’t calculate pressure drop — 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’s sizing rules take precedence.