How are UK water pipes measured?

UK copper and plastic supply pipes are sized by their outside diameter in millimetres. A “15 mm pipe” measures 15 mm across the outside, whatever the wall thickness, which is why 15 mm copper and 15 mm plastic push-fit pipe can share the same fittings. Copper tube for water and heating is made to BS EN 1057, and the sizes you will meet in a typical house are 8 or 10 mm microbore, 15 mm, 22 mm and 28 mm.

Homes plumbed before metrication may still have imperial pipe, described by size in inches. Old ½-inch pipe is close to 15 mm and ¾-inch close to 22 mm, but they are not always identical, so joining old to new can need an adaptor or a different olive. Measure across the outside with a caliper, or wrap a strip of paper round the pipe and divide its length by 3.142.

Which pipe sizes are used for what in a UK home?

Supply and heating pipes are mostly 15 mm and 22 mm, while waste pipes run from 32 mm to 110 mm. The chart below summarises typical uses. Waste and soil sizes follow Approved Document H of the Building Regulations, which sets minimum diameters: 30 mm for a basin branch, 40 mm for baths, showers and sinks, and 100 mm for a WC, so basins normally use 32 mm pipe and WCs 110 mm soil pipe.

Pipe size (outside)	Old imperial name	Typical use
8 mm or 10 mm	Microbore	Radiator connections on some heating systems
15 mm	About ½ in	Hot and cold supply to taps, toilets and radiators
22 mm	About ¾ in	Main supply runs, boiler flow and return, bath supplies
28 mm	About 1 in	Larger houses and longer supply or heating runs
32 mm waste	1¼ in	Wash basin waste
40 mm waste	1½ in	Bath, shower tray and kitchen sink waste
110 mm soil	4 in	WC connection and soil stack

For sinks, the hole in the bowl and the waste fitting are a separate question from the pipe; the sink drain size guide covers both.

What are BSP thread sizes?

BSP (British Standard Pipe) threads are named after an old nominal bore in inches, not the actual thread diameter. A ½-inch BSP thread measures about 21 mm across the outside of the thread, which surprises people who expect 12.7 mm. In UK homes, basin taps and many tap connectors use ½-inch BSP, bath taps and washing machine hoses use ¾-inch BSP, and these threads connect to 15 mm and 22 mm pipe through tap connectors or adaptors.

To identify a thread, measure the outside diameter of a male thread with a caliper and compare it with a BSP chart, or take the old part to a plumbers’ merchant. Toilet cisterns have their own sizes for inlet and flush valves, explained in the flush valve size guide.

What size electrical cable is used in UK homes?

Household cable is sized by the cross-sectional area of each copper conductor, in square millimetres. Flat twin and earth cable marked 2.5 mm² has two 2.5 mm² insulated conductors and a smaller bare earth. The figures below are typical for domestic circuits, but the correct size for any circuit depends on its load, length, protective device and how the cable is installed, following BS 7671, the IET Wiring Regulations.

Cable size	Typical domestic use
1.0 or 1.5 mm²	Lighting circuits, usually on a 6 A protective device
2.5 mm²	Socket circuits: a ring final circuit on 32 A or a radial on 20 A
4 mm²	Radial socket circuits on 32 A, some appliance circuits
6 mm² and 10 mm²	Electric cookers and electric showers, depending on rating and run length
16 mm² or 25 mm²	Meter tails between the meter and the consumer unit

Electrical work can be dangerous. In England, many jobs in homes, such as adding a new circuit or work in a bathroom, are notifiable under Part P of the Building Regulations, so they must be done by a registered competent person or notified to building control. Measuring an existing cable is safe only with the power isolated; the electrical cable measuring guide explains how to read the size printed on the sheath instead. Plug fuses are a separate matter: a 3 A fuse suits appliances up to about 700 W, and a 13 A fuse is used for higher-powered items such as kettles and heaters.

How are screw sizes described in the UK?

Wood and general-purpose screws are now sold by diameter and length in millimetres, such as 4.0 × 40 mm. The first figure is the thread diameter and the second is the length, measured from under the head for countersunk screws to the tip. Older plans and some trade catalogues still use imperial gauge numbers, which describe the diameter only.

Old screw gauge	Diameter (in)	Diameter (mm)	Nearest common metric size
No. 4	0.112 in	2.84 mm	3.0 mm
No. 6	0.138 in	3.51 mm	3.5 mm
No. 8	0.164 in	4.17 mm	4.0 mm
No. 10	0.190 in	4.83 mm	5.0 mm
No. 12	0.216 in	5.49 mm	5.5 mm

Machine screws and bolts use the ISO metric thread, written with an M: an M6 bolt has a 6 mm thread diameter, and M6 × 30 is 30 mm long. The screw size guide shows how to measure diameter, length and thread pitch.

What size spanner fits a metric nut?

A spanner is sized by the distance across the flats of the nut, not by the bolt diameter. An M8 nut needs a 13 mm spanner and an M6 nut a 10 mm spanner. For M10 and M12, two standards are in common use, so check both sizes if the first spanner is loose.

Thread	Spanner size (ISO 4032)	Spanner size (older DIN 934)
M4	7 mm	7 mm
M5	8 mm	8 mm
M6	10 mm	10 mm
M8	13 mm	13 mm
M10	16 mm	17 mm
M12	18 mm	19 mm
M16	24 mm	24 mm

Older British machinery may use Whitworth or BSF threads with their own spanner sizes, and American equipment uses fractional inch sizes. The nut size guide explains how to tell them apart.

What mistakes should you avoid when sizing hardware?

• Measuring the inside of a pipe. UK supply pipe sizes are outside diameters; an inside measurement will make 15 mm pipe look like 13 mm.
• Reading BSP sizes literally. A ¾-inch BSP thread is about 26 mm across, not 19 mm.
• Confusing cable area with diameter. 2.5 mm² is the conductor’s cross-section, not its width.
• Guessing a thread. Take the old screw, bolt or fitting to the shop, or measure it with a caliper and a thread gauge.

Plumbing and hardware guides in this section

• How to measure a screw size: diameter, length and thread pitch.
• How to measure nut size: across-flats sizes and thread identification.
• How to measure electrical cable: reading conductor sizes safely.
• How to measure flush valve size: cistern outlets and replacement valves.
• How to measure sink drain size: waste holes, fittings and pipe sizes.

For radiators and toilet seats, see the radiator measuring guide and the toilet seat measuring guide in the home section.

Latest plumbing, electrical and hardware guides

Related sections

The units of measurement guide explains the metric units behind every fitting. For measuring rooms, doors and stairs, see home and DIY measuring guides, and for calipers, tapes and thread gauges, see measuring tools and techniques.

Frequently asked questions

What size is standard UK water pipe?

Most household water and heating pipe in the UK is 15 mm or 22 mm, measured across the outside. 15 mm feeds taps, toilets and radiators, while 22 mm is used for main runs and boiler connections.

What size waste pipe does a kitchen sink need?

A kitchen sink normally uses 40 mm waste pipe, the same as a bath or shower. Wash basins use 32 mm pipe.

What cable is used for sockets in the UK?

Socket circuits are usually wired in 2.5 mm² twin and earth cable, either as a ring final circuit on a 32 A device or a radial circuit on 20 A. Larger radial circuits may use 4 mm² cable. Always have new circuits installed by a competent electrician.

What size spanner do I need for an M8 nut?

An M8 nut takes a 13 mm spanner. An M6 nut takes 10 mm, and an M10 nut takes 16 mm or 17 mm depending on the standard it was made to.

What does ½ inch BSP mean?

It is a British Standard Pipe thread named after an old nominal pipe bore of half an inch. The actual thread measures about 21 mm across the outside, and it is common on basin taps and tap connectors.

Sources

• Approved Document H: drainage and waste disposal, GOV.UK
• Approved Document P: electrical safety in dwellings, GOV.UK
• BS 7671, the IET Wiring Regulations, Institution of Engineering and Technology
• British Standards Institution (BSI), publisher of BS EN 1057 for copper tube