Calculate the correct pipe diameter for water supply, heating or combined systems based on flow rate, pipe run length, pressure and material.
The calculator uses the Darcy-Weisbach equation with the Colebrook-White friction factor to compute pressure drop, then selects the smallest standard pipe size that keeps flow velocity below the BS EN 806 recommended maximum and pressure loss within the available head.
A typical family bathroom might need around 0.3–0.5 L/s design flow to supply a bath tap, basin, and shower simultaneously. For a single en-suite, 0.2–0.3 L/s is usually adequate.
BS EN 806 recommends a maximum of 2 m/s for cold water pipework, though 1.5 m/s is preferred for quiet, long-lasting installations. For hot water, 1.0 m/s is typically recommended to reduce noise and minimise thermal expansion stress.
This is a standard allowance for fittings — elbows, tees, valves, and connectors all create additional friction losses. The 30% addition converts your measured straight-line distance into an effective pipe length that accounts for these resistances.
Copper is more durable and has a smoother bore (lower friction losses), but costs more and requires soldering. PEX and multilayer are cheaper, easier to install with push-fit fittings, and flexible enough to route through tight spaces. PVC-U is mainly for cold water supply and waste.
If you have less than 1 bar, you may need a water pressure booster pump. Alternatively, use larger diameter pipes to reduce friction losses, or minimise the number of fittings and bends on the pipe run.