Pipe Sizing Calculator — Find the Right Pipe Diameter

Our pipe sizing calculator determines the correct pipe diameter by comparing your required flow rate against the capacity of each standard UK pipe size. The calculation accounts for pipe material, run length, and friction losses from fittings.

The calculation method

Effective pipe length = Actual length + (Number of fittings x Equivalent length per fitting)

We add an indicative safety margin for longer pipe runs (a simple step-up, not a full head-loss calculation) and select the smallest standard pipe size that can deliver the required flow rate. Gas supply pipes are sized by a different method entirely (BS 6891, volumetric/pressure-drop) — choose “Gas Supply” and the tool points you to the dedicated Gas Pipe Sizing Calculator.

UK standard pipe sizes

• 10mm: Micro-bore for individual radiator feeds (central heating only)
• 15mm: Standard for individual taps, basins, and short runs
• 22mm: Main distribution, bath feeds, boiler connections
• 28mm: Large installations, long runs, multiple outlets
• 35mm: Commercial or high-demand domestic systems

Flow capacities are calculated at a default 1 m/s velocity — a conservative design figure for 15–50mm copper (CIBSE Guide B1 (2016) Table 1.A1.4, aligned to BSRIA BG 4/2007 Design Checks for HVAC) that limits noise and erosion in copper. It is not the standard’s ceiling: BS EN 806 (which superseded BS 6700) permits up to about 3 m/s in cold-water pipes and 2.5 m/s in hot, but designing to 1 m/s keeps systems quiet and oversizes safely. You can raise the design velocity in Advanced Options. Gas pipe sizing follows BS 6891 and must be confirmed by a Gas Safe registered engineer.