Copper Pipe Sizes and Fittings — The Complete UK Guide
UK domestic plumbing uses four copper pipe sizes: 10, 15, 22 and 28mm outside diameter (BS EN 1057, table X). Pick the wrong size and you throttle flow, waterhammer or oversize a job. Here is the definitive sizing chart plus fitting compatibility.
Step-by-step method
10mm — Microbore heating drops
Used for feeds to individual radiators, especially upstairs. Max flow ~5 l/min. Neat and easy to hide, but never for hot or cold mains.
15mm — The workhorse
Standard for hot and cold feeds to basins, toilets, showers (with a good pressure), and most radiators. Max flow ~15 l/min. 90% of house pipework is 15mm.
22mm — Bath, boiler feed, main runs
Required for bath fills (30+ l/min), combi cold feeds, main flow/return between manifold and radiators. Also mandatory to boiler condensate drain runs of 3m+.
28mm — Mains and commercial
Incoming mains, unvented cylinder feeds, and larger commercial jobs. Flow to 60+ l/min. Not needed in most homes but common at the stopcock.
Endfeed vs Solder-ring
Endfeed (Yorkshire style) needs added solder wire — cheaper, needs skill. Solder-ring has integral lead-free solder — pricier but idiot-proof and consistent. Both are permanent, WRAS-approved for potable water.
Compression fittings
Brass body, brass olive, compression nut. No heat needed. Use where flames aren't allowed (near joists, under floors, in occupied lofts). Slightly bulky but demountable.
Press-fit (Conex B-Press, Xpress)
Modern trade favourite. Jaws crimp a stainless sleeve onto an O-ring — 3 seconds per joint, 100% repeatable. £600+ tool cost but pays back on any 5+ day job.
Frequently asked questions
Can I mix 15mm and 22mm pipe?+
Yes, with a reducing coupler (22→15). Common where a 22mm main splits into 15mm radiator drops.
Is copper still legal for drinking water in the UK?+
Yes, WRAS-approved and BS EN 1057. Modern lead-free solders (95% tin) are mandatory — old plumbo-tin solder is not permitted on potable systems.