What size pipe do I need for a combi boiler?

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

What is the standard pipe size for domestic hot water in the UK?

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.

Does pipe material affect flow rate?

Yes. Copper pipe has a slightly larger internal diameter than PEX/plastic of the same nominal size, so copper generally has higher flow capacity. For example, 15mm copper has an internal bore of ~13mm, while 15mm PEX is ~10mm internally. This means you may need the next size up in plastic for the same flow rate.

How do pipe fittings affect sizing?

Every elbow, tee, and valve creates friction that reduces flow — this is known as "equivalent pipe length." Each fitting adds roughly 0.5-1.0m of equivalent resistance. Our calculator accounts for this by adding the equivalent length of your fittings to the total pipe run.

Can I use 15mm pipe for a shower?

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

Pipe Sizing Calculator — What Size Pipe Do I Need for Plumbing?

How We Calculate This

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 then factor in friction losses for longer pipe runs and select the smallest standard pipe size that can deliver the required flow rate. For gas supply pipes, additional derating is applied per BS 6891.

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 1 m/s velocity (the recommended maximum for domestic hot and cold water per BS 6700). Gas pipe sizing follows BS 6891 guidelines and must be confirmed by a Gas Safe registered engineer.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Pipe Sizing Calculator — Find the Right Pipe Diameter