Pipe Diameter Calculator

Every pipe question comes down to one small equation: how much water must move, and how fast its allowed to travel. Answer those two and the bore that joins them is fixed.

Pipe Diameter Calculator solves either direction - the internal diameter a flow needs at your design velocity, or the flow a chosen pipe will carry - in metric or imperial units.

It works on the true internal bore, not the nominal label, so results transfer across materials.

• Two directions: Find diameter from flow, or capacity from diameter.
• Velocity control: Size to your design velocity.
• Full units: GPM, LPM, m/s, fps, mm and inches.
• Nominal hint: Nearest common nominal size shown.
• Instant results: Live computation as you type.
• Export: Copy or download as text, CSV or JSON.

1. Pick the direction: diameter from flow, or capacity from diameter.
2. Enter flow (GPM/LPM) and target velocity, or the pipe diameter and velocity.
3. Choose the unit system.
4. Read the result in mm and inches.
5. Use it with the material bore tables for a nominal size.

Example 1 — Demand sizing. 6 GPM at 4 fps needs about a 0.78 in bore - a 1 inch pipe once you honor the internal bore.

Example 2 — Capacity. A 3/4 in internal pipe at 4 fps carries roughly 5.5 GPM.

Example 3 — Metric. 22 LPM at 1.2 m/s requires about a 20 mm internal bore.

• Quick bore math: At the layout, not in the manual.
• Both directions: Sizing and capacity checks.
• Any units: Metric and imperial mixed inputs.
• Material-neutral: Works on true internal size.
• Free and private: No uploads, no logging.