Quick answer: UK copper and plastic water pipes are sized by their outside diameter in millimetres, so measure across the outside of the pipe. A caliper gives the most accurate reading, or wrap a strip of paper round the pipe and divide its length by 3.142. A circumference of about 47 mm means 15 mm pipe; about 69 mm means 22 mm.
Getting the pipe size right matters before you buy a single fitting, because a 15 mm compression fitting will not grip 22 mm pipe and an imperial pipe may need an adaptor. This guide covers measuring methods, a size-identification chart and the pipes you are likely to find in a UK home. For the wider picture, see our guide to plumbing and hardware sizes.
How are pipes measured in the UK?
Modern copper and plastic supply pipes are measured by outside diameter (OD), while older steel pipes and threaded fittings use inch sizes based on a nominal bore. This split catches many people out. A “15 mm” copper pipe really is 15 mm across the outside, but a “½ inch” steel pipe is about 21.3 mm across, because its name refers to an old nominal bore rather than any measurement you can take with a ruler.
Copper tube for water and heating is made to BS EN 1057. Plastic push-fit systems for supply and heating are made to the same outside diameters, so 15 mm plastic and 15 mm copper share fittings, although plastic pipe needs a pipe insert in most fittings.
What do you need to measure a pipe?
- A vernier or digital caliper, the most accurate option for pipes up to about 150 mm
- A strip of paper or thin card about 15 mm wide, and a pencil
- A steel rule or tape measure with millimetre markings
- A torch for pipes under sinks and in airing cupboards
- A small magnet, which helps tell steel pipe from copper or lead
How do you measure pipe diameter step by step?
Measure the bare outside of the pipe at a straight section, away from fittings. Use whichever of these methods suits the tools you have.
With a caliper
- Clear the pipe. Pull back any lagging and wipe off paint drips or limescale.
- Close the outer jaws on the pipe. Hold the caliper square to the pipe, not at an angle, and close the jaws gently.
- Read the scale. Note the figure to a tenth of a millimetre if you can.
- Turn and repeat. Rotate the caliper a quarter turn and measure again; old pipe can be slightly oval.
With a paper strip
- Wrap the strip. Wind it once round the pipe, snug but not stretched.
- Mark the overlap. Draw a pencil line across both layers where the end meets the strip.
- Measure the strip. Unwrap it and measure between the end and the mark in millimetres. That is the circumference.
- Divide by π. Circumference ÷ 3.142 = outside diameter. Then match it to the nearest size in the chart.
Holding a tape measure straight across a pipe is the least accurate method, because it is hard to judge the widest point of a curve. If you do use one, our guide to reading a tape measure explains the millimetre markings.
What size is my pipe? UK identification chart
Match your measured outside diameter or circumference to the nearest row below, then check the material. Circumferences are π × the outside diameter, rounded to 0.1 mm.
| Measured OD | Circumference | Most likely pipe | Where you find it |
|---|---|---|---|
| 8 mm | 25.1 mm | 8 mm microbore copper | Radiator connections on some heating systems |
| 10 mm | 31.4 mm | 10 mm microbore copper or plastic | Radiator connections, some appliance feeds |
| 15 mm | 47.1 mm | 15 mm copper or plastic | Taps, toilets, radiators |
| 20 mm | 62.8 mm | 20 mm MDPE (blue plastic) | Underground water supply into the house |
| About 21.3 mm | 66.9 mm | ½ inch steel (BSP nominal bore) | Older gas and heating pipework |
| 22 mm | 69.1 mm | 22 mm copper or plastic | Main supply runs, boiler flow and return |
| 25 mm | 78.5 mm | 25 mm MDPE (blue plastic) | Underground water supply |
| About 26.9 mm | 84.5 mm | ¾ inch steel | Older gas and heating pipework |
| 28 mm | 88.0 mm | 28 mm copper | Larger heating and supply runs |
| 32 mm | 100.5 mm | 32 mm MDPE (blue plastic) | Larger underground supplies |
| 110 mm | 345.6 mm | 110 mm soil pipe | WC connection and soil stack |
Notice how close ½ inch steel (about 21.3 mm) is to 22 mm copper. If your reading falls between the two, check the material: steel is grey, attracts a magnet and joins with threaded fittings, while copper is non-magnetic and usually has compression or soldered joints.
How do you measure waste pipes?
Plastic waste pipes are sold by nominal sizes of 32 mm, 40 mm and 50 mm, and their actual outside diameter is a little larger. One UK merchant lists its universal 32 mm waste pipe as about 34.5 mm across the outside, for example. Basins normally use 32 mm waste, while baths, showers and kitchen sinks use 40 mm. Push-fit and solvent-weld systems are not always interchangeable, so measure the outside of the existing pipe and check it against the manufacturer’s figures.
For the outlet in the bowl itself, rather than the pipe beneath, see our sink drain size guide. Soil pipes for WCs are 110 mm outside diameter, often still called 4 inch.
Will imperial pipe fit metric fittings?
Old ½ inch copper is very close to 15 mm and can usually be joined as if it were 15 mm, but larger imperial sizes need adaptors. Houses plumbed before metrication, around the early 1970s, may still have imperial copper. Imperial ¾ inch and 1 inch tube are slightly larger than 22 mm and 28 mm, so use an imperial-to-metric adaptor or a fitting designed to take both. If you are not sure which you have, measure with a caliper rather than guessing.
Threaded fittings are a separate case. BSP (British Standard Pipe) threads are named after old nominal bores: a ½ inch BSP male thread measures about 20.96 mm across the outside of the thread, and a ¾ inch BSP thread about 26.44 mm. Our inches to cm converter helps when old drawings give sizes in inches.
How can you tell if a pipe is lead?
Lead pipe is dull grey, soft enough to scratch easily to a shiny silver colour, does not attract a magnet and often has rounded, swollen joints. Homes built before about 1970 may still have lead supply pipes. Water supplied through lead pipes can be bad for your health, so if you find any, contact your water supplier for advice and use an approved plumber to replace it with copper or plastic. Wash your hands after touching or scratching it.
Gas pipework is also copper or steel in many homes. You can measure it, but only a Gas Safe registered engineer may work on it.
Common mistakes to avoid
- Measuring the inside. A bore measurement of about 13.6 mm belongs to 15 mm pipe, not a 13 mm or 14 mm size.
- Measuring over lagging or paint. Thick paint or tape can add enough to make a 15 mm pipe read oversize.
- Reading inch sizes literally. A ½ inch BSP thread or steel pipe is far wider than 12.7 mm.
- Measuring at a fitting. Nuts, olives and solder rings are larger than the pipe. Measure on a plain straight length.
Frequently asked questions
Are UK pipe sizes inside or outside diameter?
UK copper and plastic supply pipes are sized by outside diameter, so 15 mm pipe measures 15 mm across the outside. Steel pipe and BSP threads use inch sizes based on an old nominal bore instead.
How do I measure a pipe without a caliper?
Wrap a strip of paper round the pipe, mark where it overlaps, measure the length in millimetres and divide by 3.142. A strip of about 47 mm means 15 mm pipe, and about 69 mm means 22 mm pipe.
What is the inside diameter of 15 mm copper pipe?
With the common 0.7 mm wall thickness, 15 mm copper pipe has an inside diameter of about 13.6 mm. 22 mm pipe with a 0.9 mm wall has a bore of about 20.2 mm.
Is ½ inch pipe the same as 15 mm?
Old ½ inch imperial copper is close enough to 15 mm to be joined with 15 mm fittings in most cases. A ½ inch steel pipe or BSP thread is much larger, at about 21 mm across.
What size is the blue water pipe coming into my house?
Blue MDPE supply pipe is usually 20 mm, 25 mm or 32 mm outside diameter. Measure it where it enters the house, near the internal stop tap.
Sources
- EN 1057 copper tubes: dimensions, Engineering ToolBox
- ISO 228-1: Pipe threads where pressure-tight joints are not made on the threads, ISO
- Lead in water, WaterSafe
- Lead pipes, NI Water
- Gas Safe Register