Pipe Sizing Calculator — Find the Right Pipe Diameter

Most combi boilers in the UK require 22 mm flow and return pipes for central heating, and 15 mm for individual radiator feeds. The cold mains feed to the boiler should typically be 22 mm for adequate flow. Always check the boiler manufacturer specifications — use our Boiler Size Calculator to determine the right kW output first. For waste pipe runs, see our Waste Pipe Sizing Calculator, and for heating pipe runs, our Radiator Pipe Sizing Calculator.

In UK domestic plumbing, 15mm copper or plastic pipe is standard for individual hot and cold water outlets (taps, showers). 22mm is used for main distribution runs and feeds to baths and combi boilers. 28mm is used for larger installations or long pipe runs.

Yes. Copper pipe has a larger internal bore than PEX/plastic of the same nominal size, so copper generally has higher flow capacity. For example, 15mm copper has an internal bore of about 13.6mm (BS EN 1057 Table X), while 15mm PEX is roughly 10mm internally. This means you may need the next size up in plastic for the same flow rate.

Every elbow, tee, and valve creates friction that reduces flow — this is known as "equivalent pipe length." As a rule of thumb each small-bore fitting adds roughly 0.5-1.0m of equivalent resistance (the true figure scales with bore and fitting type — check fitting data where precision matters). Our calculator approximates this by adding a fixed equivalent length per fitting to the total pipe run; you can override the figure in Advanced Options.

A 15 mm pipe is fine for a single mixer shower in most UK homes where the pipe run is under 10 m. For power showers, multiple showers, or long pipe runs, 22 mm is recommended to avoid pressure drop. Electric showers typically only need 15 mm cold feed. Check your available pressure with our Water Pressure Calculator.

No. Water and central heating pipes are sized here by matching flow rate against pipe capacity at a design velocity. Gas pipework uses a completely different method: BS 6891 sizes gas pipe by pressure drop, allowing a maximum of 1 mbar drop between the meter and the appliance. Selecting “Gas Supply” above points you to the Gas Pipe Sizing Calculator, which follows BS 6891:2015 Annex A for copper and steel pipework up to 35 mm (larger commercial runs fall under IGEM/UP/2). All gas pipework must be installed and signed off by a Gas Safe registered engineer.

Choosing “Central Heating” above sizes pipe by flow rate and velocity, the same water-based method this calculator uses for hot and cold water, with a conservative default design velocity of 1.0 m/s (CIBSE Guide B1 Table 1.A1.4). For heating pipework specifically, CIBSE Guide G caps design velocity lower than the potable-water ceilings in BS EN 806, at around 1.5 m/s, so keep Max Flow Velocity in Advanced Options at or below that for heating runs. If you know each radiator's BTU or kW load rather than a target flow rate, the Radiator Pipe Sizing Calculator sizes each pipe section by cumulative load against the HVCA Central Heating Design Guide quick-sizing table (15 mm rated to 6 kW, 22 mm to 13 kW, at an 11 °C temperature drop) and shows exactly where a run can step down from 22 mm to 15 mm.

UK copper and plastic pipe is sized by outside diameter, not internal bore, so a “15 mm pipe” measures 15 mm across the outside (BS EN 1057) whether it is copper or push-fit plastic. To measure an existing pipe, wrap a tape measure around it and divide the circumference by π (3.142) to get the diameter, or measure straight across the cut end with a vernier caliper. Round to the nearest common size: 15 mm, 22 mm and 28 mm cover most UK domestic supply and heating pipework, with 10 mm micro-bore used for individual radiator feeds. See our UK Copper Pipe Sizes reference for the full outside diameter and wall thickness table, from 8 mm micro-bore up to 54 mm commercial sizes.

The underground service pipe from the water main to a UK domestic property is almost always 25 mm blue MDPE (medium-density polyethylene). This calculator's MDPE option covers 20, 25, 32, 50 and 63 mm: 20 mm for short single-connection runs, 25 mm as the standard domestic service pipe, and 32 mm upward for larger, shared or commercial supplies. Once inside the property, the cold mains feed typically continues in 15 mm or 22 mm copper or plastic, with 22 mm giving more headroom for flow rate into a combi boiler. See our UK Pipe Sizes reference for more on MDPE service pipe.

This tool's smallest listed size is 10 mm. 8 mm copper micro-bore also exists and is sized the same way as 10 mm: for an individual radiator feed only, never a shared run carrying multiple radiators. If you are sizing micro-bore by radiator BTU or kW load rather than by target flow rate, the HVCA Central Heating Design Guide quick-sizing table caps 8 mm at 1.5 kW and 10 mm at 2.5 kW at an 11 °C temperature drop. Our Radiator Pipe Sizing Calculator applies those limits directly and only ever offers micro-bore for the single furthest radiator branch.