The short answer
Use 15mm pipe for single outlets — basin and kitchen taps, toilets, radiator tails and most showers on a combi. Use 22mm for main runs feeding several outlets, bath taps, gravity-fed and pumped showers, and the heating flow and return near the boiler. Bigger pipe carries more water with less pressure lost, which matters most on low-pressure systems. 28mm is for larger homes and commercial systems.
What each size is used for
| Size | Typical uses |
|---|---|
| 8mm and 10mm (microbore) | Radiator connections on older microbore heating systems |
| 15mm | Basin, sink and toilet supplies; radiator tails; most mixer showers on mains pressure; hot and cold to a combi |
| 22mm | Main hot and cold runs; bath taps; gravity-fed and pumped showers; heating flow and return near the boiler; often the gas supply to a boiler |
| 28mm | Larger heating systems and main runs in bigger homes and commercial buildings |
| 25mm or 32mm MDPE (blue) | The underground mains water supply into the house |
Why bigger pipe gives more flow
Water loses pressure to friction as it travels, and the loss is much greater in a narrow pipe. A 22mm pipe has roughly twice the bore of a 15mm one, so it carries far more water over the same distance for the same pressure drop.
On mains pressure (a combi or unvented cylinder) 15mm is usually fine for single outlets. On a gravity system, where a tank in the loft gives perhaps a few tenths of a bar, 22mm to the bath and shower makes a real difference — and long runs in 15mm will starve the far end.
Showers, baths and combis
- Mixer shower on a combi or unvented cylinder: 15mm supplies are usually enough; run 22mm for long distances or a big rainfall head.
- Gravity-fed or pumped shower: 22mm from the tank and cylinder, reducing to 15mm close to the valve or pump if its fittings need it.
- Bath taps: 22mm fills the bath noticeably faster, especially on gravity.
- Combi boiler: hot and cold connections are typically 15mm; heating flow and return 22mm. Gas pipe sizing is for the Gas Safe engineer — bigger boilers often need 22mm gas.
If the shower is weak, see low water pressure: taps and showers before re-piping.
Is 15mm the same as 1/2 inch?
Not exactly. Metric copper is measured across the outside; old imperial copper was named by size in inches. Old 1/2" imperial is close enough to 15mm that compression fittings normally work. 3/4" imperial is larger than 22mm, so it needs an imperial olive or a 22mm × 3/4" adaptor. For soldered joints, use the matching imperial-to-metric adaptor fittings.
Don't confuse pipe size with thread size: a 15mm × 1/2" fitting joins 15mm pipe to a 1/2" BSP thread, which is about 21mm across. Our BSP thread sizes guide explains the difference.
Pipe on our shelves
| Pipe | Lines | From |
|---|---|---|
| Copper tube | 16 | £18.92 |
| Plastic pipe | 37 | £9.21 |
| MDPE mains pipe | 13 | £1.07 |
| Pipe insulation | 41 | £1.25 |
Questions
- Should I use 15mm or 22mm pipe for a shower?
- On a combi or unvented cylinder, 15mm is usually fine. On a gravity-fed or pumped system, use 22mm for the main runs to get enough flow.
- Is central heating pipe 15mm or 22mm?
- Both: 22mm for the flow and return mains from the boiler, 15mm (or 8–10mm microbore on older systems) for the drops to each radiator.
- Can I reduce from 22mm to 15mm?
- Yes, with a reducer or a reducing tee — it's normal to run 22mm and reduce to 15mm for each outlet. Avoid long 15mm runs feeding several outlets.
- Is 1/2 inch pipe the same as 15mm?
- Close enough for compression fittings in most cases, but not identical. Solder joints need an imperial-to-metric adaptor.
Related guides
- Bathrooms & showersLow Water Pressure: Which Taps and Showers Will Work?