Pick the wrong diameter on a heating circuit and you get noisy pipes, weak flow at the far radiators and callbacks. Get it right and the system runs quiet and balanced. Knowing the copper pipe sizes UK merchants actually stock, and what each one is built for, is basic groundwork for any plumber or heating engineer.
How copper pipe is sized in the UK
British copper tube is metric, and it’s measured on the outside diameter. A 15mm copper pipe measures 15mm across the outside, not the bore. That matters because fittings grip the outer wall, so the stated size is exactly what your compression or press fitting is cut to.
The governing standard is BS EN 1057, which covers seamless copper tube for water and gas in sanitary and heating work. It replaced the older BS 2871 Part 1. Tube made to this standard uses phosphorus deoxidised copper, the grade that solders cleanly and resists corrosion in domestic systems.
You’ll hear the figure called the nominal diameter. For copper tube that number is the real outside diameter, so a nominal 22mm copper pipe is 22mm OD. Plastic and steel don’t always follow that logic, which is why mixing materials needs the correct transition fitting.
The copper pipe sizes UK merchants stock
Merchants carry a fixed range off the shelf. This quick copper pipe sizes chart shows the diameters you’ll handle on most jobs and the work each one tends to do:
| Size (OD): | Common use: |
|---|---|
| 6mm, 8mm, 10mm: | Microbore. Radiator tails, gas supply to a single appliance, tight runs. |
| 12mm: | Some microbore heating and short appliance connections. |
| 15mm: | Hot and cold supply to taps, showers, basins and radiators. The most common size. |
| 22mm: | Main supply runs, bath cold feeds, heating flow and return on smaller systems. |
| 28mm: | Boiler connections, flow and return on larger heating, hot water cylinder feeds. |
| 35mm, 42mm, 54mm: | Larger heating circuits, commercial plant rooms, riser mains. |
| 66.7mm to 159mm: | Commercial and industrial pipework, beyond domestic work. |
The smallest three sizes, 6mm, 8mm and 10mm, are microbore, sold in soft coils you can form by hand. Merchant stock for plumbing runs 15, 22, 28, 35, 42, 54, 66.7, 76.1, 108, 133 and 159mm, and you’ll often see 66.7 and 76.1 labelled 67 and 76 on price lists.
Wall thickness and temper: Table X, Y and Z
Diameter is only half the spec. Wall thickness and temper decide how the tube behaves when you bend it and how much pressure it carries.
The old BS 2871 tables
The trade still uses the Table names from BS 2871, even though BS EN 1057 took over:
- Table X: Half-hard, light gauge. This is the default for domestic plumbing and gas, sold in straight lengths, and it covers most of what you fit day to day.
- Table Y: Thicker wall, half-hard or annealed, available in coils. It suits underground runs and gas work where a tougher wall earns its place.
- Table Z: Thin wall, hard drawn. It holds higher pressure for its weight but kinks if you bend it, so it's cut and jointed rather than formed.
The BS EN 1057 tempers
The current standard describes the same idea through temper codes, and manufacturers like Wednesbury publish the full wall thickness and temper tables for each size:
- R220: Soft annealed, supplied in coils, bends by hand. This is the microbore and underground choice.
- R250: Half-hard, straight lengths. This is the common merchant stock for general plumbing.
- R290: Hard, straight lengths. It's stiffer and stronger, and not for bending.
The two systems line up on the shelf. Table X tube is R250 in the common sizes from 15mm to 54mm, and R290 in the larger 67mm and above.
Matching copper pipe sizes to the job
Sizing is about flow and pressure drop, not habit. Choosing between the common copper pipe sizes UK plumbers fit comes down to the duty each run carries.
Water supply
15mm copper pipe carries hot and cold to most fittings. Step up to 22mm on long runs or where several outlets draw at once, so flow holds up. A bath cold feed is often 22mm to fill at a sensible rate.
Central heating
Radiator tails are usually 15mm, or 8mm and 10mm microbore on a manifold layout. The main flow and return off the boiler run in 22mm or 28mm copper pipe, dropping to 15mm at the branches. Undersized heating pipe starves the far radiators and turns balancing into a fight.
Gaz
Domestic gas pipework follows BS 6891, which covers low pressure installations up to 35mm. Most boiler connections are made in 22mm or 28mm copper. The run from meter to appliance has to stay within a 1 mbar pressure drop, so the pipe is sized on gas rate and length, never guessed. Gas work is for Gas Safe registered engineers only.
Old imperial pipe and joining it to metric
Plenty of older houses still run imperial copper, so you’ll meet it on refurbs and repairs. Britain switched copper tube from imperial to metric in the early 1970s. The older sizes were set on the internal bore and the metric ones on the outside, which changes how they pair up:
- Half inch: Close enough to 15mm to join with standard metric fittings, though an imperial olive gives a cleaner seal on a compression joint.
- Three-quarter inch: Nearer 21.5mm than 22mm, so it needs the correct three-quarter inch to 22mm adaptor or imperial olive.
- One inch: Pairs with 28mm, again using the right adaptor or olive rather than forcing a metric fitting on.
Forcing a metric fitting onto imperial tube is where leaks start. Carry a few imperial olives and conversion couplers if the property predates the 1970s.
Understanding pipe sizes is a useful starting point. Keep learning with TradeFox’s plumbing courses, where guided simulations and feedback let you work through each lesson at your own pace and revisit it when needed.
Staying compliant and safe
Pipe sizing isn’t only about performance. On water and gas it carries a legal duty, and the rules change across the UK.
Water regulations
Plumbing that carries wholesome water has to meet the water fittings rules for the nation you’re working in:
- England and Wales: The Water Supply (Water Fittings) Regulations 1999.
- Scotland: The Water Supply (Water Fittings) (Scotland) Byelaws 2014.
- Northern Ireland: The Water Supply (Water Fittings) Regulations (Northern Ireland) 2009.
All three set the standard for materials, installation and protecting the supply from contamination. Copper buried in the ground should be plastic coated so it resists corrosion.
Gas regulations
Gas is stricter again. The law requires anyone working on gas fittings to be on the Registre Gas Safe, a duty set out in the Gas Safety (Installation and Use) Regulations 1998. Sizing to BS 6891 and jointing with lead-free solder are part of a safe, legal install.
Across water, heating and gas, the copper pipe sizes UK installers reach for are chosen on duty, flow and the rules that apply, so the finished job runs safely, quietly and is signed off.
