Sizes pipe by velocity using Q = A × v. Outputs a required internal diameter and nearest
nominal size from reference tables.
Tender-stage only. Verify with manufacturer data, pressure drop, and project spec.
Pipes must satisfy both v ≤ velocity limit AND ΔP/m ≤ pressure-drop limit.
For design guidance only. Always verify with a qualified engineer and project spec.
About this pipe sizing calculator
This free pipe sizing calculator sizes water pipework by velocity, returning the required internal diameter and the nearest nominal size from reference tables for copper (EN 1057), steel (BS EN 10255) and MDPE (PE80/PE100). It is built for UK building-services engineers, plumbers and mechanical contractors needing a quick tender-stage check on hot, cold and potable water services. Enter a flow rate in L/s, L/min or m³/h, choose a target velocity, and it suggests a size while flagging velocity and pressure-drop limits. It is a design aid only. Always verify against manufacturer data, full pressure-drop analysis and the project specification.
How it works
Sizing follows the continuity relationship Q = A·v, solved for the required internal diameter as D = √(4Q/πv), with an optional round-up percentage for headroom. Pressure drop per metre is computed with the Darcy-Weisbach equation, using a Colebrook-White (Moody) friction factor. Water viscosity is taken by temperature and the wall roughness ε by pipe material. The calculator then picks the smallest standard size whose internal bore meets the required diameter while keeping velocity within your chosen limit and ΔP/m at or below the pressure-drop cap (CIBSE typical 200–300 Pa/m). Calculations follow CIBSE Guide G §3.4 and BS EN 806-3.
Frequently asked questions
Which pipe materials are supported?
Copper to EN 1057, steel to BS EN 10255 and MDPE (PE80/PE100), each with its own nominal-size table and wall roughness used in the friction calculation.
What velocity should I target?
Presets range from 0.6 m/s (quiet/domestic) to 2.0 m/s (upper end, check noise), with 1.0 m/s as a general rule of thumb and a custom option. A separate warning limit flags velocities above 1.5 or 2.0 m/s.
Does it warn about Legionella or erosion?
Yes. For HWS/CHW and cold-water services it flags velocities below 0.7 m/s (sediment/Legionella risk), and warns where copper hot circuits above 60 °C exceed 1.0 m/s, risking erosion-corrosion (BS 6700).