Pipe Size Calculator (Flow Rate / Fixture Units)

A pipe is a compromise written in millimetres: too thin and the fixtures run dry at the far end; too fat and you pay for copper or plastic that does nothing but fill. The right size juggles flow, pressure, velocity and the fixture demand.

Pipe Size Calculator turns a flow rate - or a total fixture-unit count - into a required internal diameter, then suggests the nominal pipe size for your material at a chosen velocity, complete with a friction-loss check.

It is a planning aid for residential, commercial and supply piping. Final sizing follows local code.

• Flow or fixture units: Size from GPM/LPM or total FU.
• Hunter curve: FU converted to probable flow.
• Target velocity: Set your design velocity in m/s or fps.
• Material aware: PEX, copper, PVC, CPVC bores.
• Metric & imperial: Full unit support.
• Friction check: Hazen-Williams loss per 100 ft.
• Export: Copy or download as text, CSV or JSON.

1. Enter the flow rate (or fixture-unit total).
2. Choose the source type: flow or fixture units.
3. Set the target water velocity.
4. Pick the pipe material for the nominal table.
5. Optional: add length for the friction check.
6. Read the required bore and suggested nominal size.

Example 1 — Bathroom group. 8 fixture units, 0.9 m/s target: about 4.8 gpm probable flow, requiring roughly a 3/4 inch copper or 15 mm PEX run.

Example 2 — 3/4 inch PEX. At 2 m/s that bore carries about 9.3 gpm before velocity limits are hit.

Example 3 — Mains riser. 15 gpm at 1.2 m/s needs an internal bore of about 1.4 inches - a 1-1/2 inch supply.

• Right-sized runs: No more over-boring or starved fixtures.
• Both inputs: Flow or fixture units.
• Velocity aware: Avoid noise and erosion.
• Material honest: Real bores per material.
• Free and private: No uploads, no logging.