Copper Pipe Sizing Chart for Plumbers
Copper pipe is sized by its outside diameter, and each size has a job it does well. Pick the size to suit the flow and the run, and the system behaves — get it wrong and you meet pressure loss, water hammer or dead legs.
Most domestic plumbing runs on a handful of copper sizes, and knowing which one belongs where is half the battle. The chart below is a quick reference for the common UK sizes and what each is typically used for on hot and cold supply and on central heating.
Treat the flow figures as a rough guide, not a specification. Real sizing depends on the water pressure you have, how long the pipe run is and how many outlets might draw at once — so use the chart to narrow the choice, then check it against the job.
Key takeaways
- UK copper pipe is sized by outside diameter — 15mm, 22mm and 28mm cover most domestic work.
- 15mm feeds a single tap or appliance; 22mm carries a bath or several outlets; 28mm and up are for main runs.
- Central heating flow and return are commonly 22mm and 28mm, dropping to 15mm at individual radiators.
- Undersizing starves outlets and causes pressure loss and noise; oversizing wastes money and creates dead legs.
- The chart narrows the choice — pressure, run length and simultaneous demand settle it.
How copper pipe is sized in the UK
UK copper pipe is specified by its outside diameter (OD) in millimetres — 15mm, 22mm, 28mm and so on — which is why a fitting stamped 15mm matches the outside of the pipe, not the bore. This differs from the old imperial half-inch and three-quarter-inch sizing, and from some overseas systems that quote the inside diameter, so it pays to be clear which convention you are working to.
The size you choose sets how much water the pipe can carry at a given pressure. A larger bore moves more water with less friction, but costs more in pipe, fittings and insulation and holds more standing water — so the aim is the smallest size that comfortably meets the demand at the far end of the run.
Common copper sizes and what they are used for
The chart below sets out the everyday copper sizes and the work each typically does. The flow column is a qualitative guide to the sort of demand a size suits — a single outlet, a bath or several outlets, or a main run — rather than a precise flow rate, because the real figure moves with pressure and pipe length.
In most homes the great majority of visible pipework is 15mm and 22mm, with 28mm appearing on incoming mains, cylinder connections and heating flow and return. The larger sizes are far more common on commercial and plant work than on a typical house.
| Pipe size (OD) | Typical use | Approx flow guide |
|---|---|---|
| 15 mm | Feeds to individual taps, basins, WCs and single appliances; radiator tails | Suits a single tap or appliance |
| 22 mm | Baths, showers, runs serving several outlets, cylinder and boiler connections | Suits a bath or a few outlets together |
| 28 mm | Main hot and cold distribution, heating flow and return, larger cylinder feeds | Suits main runs and multiple outlets |
| 35 mm | Larger dwellings and small commercial main runs and headers | Suits heavier main runs |
| 42 mm | Commercial risers, plant and boosted distribution | Suits high simultaneous demand |
| 54 mm | Large commercial mains, plant room headers and boosted systems | Suits large main runs and plant |
Cold, hot and central-heating pipework
On cold supply the incoming main is often 22mm or 28mm, branching to 15mm at each outlet. Hot pipework follows a similar logic but keeps runs to outlets as short as sensible, because every extra metre of 22mm hot pipe is more water to draw off — and more wait — before the tap runs hot.
Central heating is a closed loop, so it is sized for flow rather than draw-off: the flow and return from the boiler or cylinder coil are commonly 22mm or 28mm, reducing to 15mm tails at individual radiators. Getting the main heating runs right keeps the circulation balanced so distant radiators still heat properly.
Why undersizing and oversizing both cause problems
Undersize a pipe and the flow it can deliver falls short: outlets run weakly, pressure drops noticeably when a second tap opens, and the higher water velocity can bring water hammer and noise. Feed a bath off 15mm and the fill is slow; run a shower off a starved branch and it dies every time the kitchen tap opens.
Oversizing looks safe but has its own costs. Bigger pipe and fittings cost more, and the larger volume of standing water means longer waits for hot water and more scope for dead legs — sections where water sits and stagnates. The right size is the smallest that meets the demand comfortably, not the largest that will fit.
Size for the far end of the run
Check the pipe delivers at the outlet furthest from the source, under the pressure you actually have. A size that works next to the cylinder can still disappoint at the top of the house.
Frequently asked questions
Is copper pipe sized by inside or outside diameter?
In the UK copper pipe is sized by its outside diameter — 15mm, 22mm, 28mm and so on. Fittings are specified the same way, which is why a 15mm fitting matches the outside of 15mm pipe rather than its bore.
What is 15mm copper pipe used for?
15mm is the everyday size for feeds to individual taps, basins, WCs and single appliances, and for the tails to individual radiators. It suits a single outlet; a bath or several outlets together usually wants 22mm.
What size copper pipe is used for central heating?
Heating flow and return from the boiler or cylinder are commonly 22mm or 28mm, reducing to 15mm at individual radiators. The exact size depends on the system's flow rate and layout, so treat these as a starting point.
Can I use a bigger pipe to be safe?
Oversizing costs more in pipe and fittings and holds more standing water, which means longer waits for hot water and a greater risk of dead legs. Aim for the smallest size that meets the demand comfortably rather than the largest that fits.
From guidance to action
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