Quick answer: UK basin and kitchen sink pillar taps normally have ½ inch BSP tails and bath pillar taps ¾ inch BSP. Measure across the thread of the tail: about 21 mm means ½ inch and about 26 mm means ¾ inch. Monobloc mixers use flexible tails instead, so for those measure the tap hole, deck thickness and spout reach.
“Tap size” can mean four different things: the thread on the tail, the size and number of holes in the sink or bath, the distance between those holes, and the reach and height of the spout. Get all four right and a new tap will fit first time. This guide is part of our plumbing and hardware sizes section.
What size are UK tap tails?
Basin and kitchen sink pillar taps use ½ inch BSP tails, and bath pillar taps use ¾ inch BSP tails. BSP stands for British Standard Pipe, a parallel thread now defined in ISO 228-1. The inch name refers to an old nominal pipe bore, not the size you can measure, which is why a ½ inch thread is about 21 mm across.
| Thread size | Measured across the thread | Threads per inch | Where you find it at home | Usual pipe connection |
|---|---|---|---|---|
| ½ in BSP | 20.96 mm (about 21 mm) | 14 | Basin and kitchen sink pillar taps, many isolating valves | 15 mm, via a ½ in tap connector |
| ¾ in BSP | 26.44 mm (about 26 mm) | 14 | Bath pillar taps, washing machine valve outlets | 22 mm, via a ¾ in tap connector |
| 1 in BSP | 33.25 mm (about 33 mm) | 11 | Larger fittings, rarely on household taps | Larger pipework |
Because ½ and ¾ inch threads have the same 14 threads per inch, you cannot tell them apart by the thread spacing alone. Measure the diameter. The inches to cm converter is handy if an older tap’s paperwork gives sizes in inches.
What do you need to measure a tap?
- A caliper, or a steel rule with millimetre markings
- A tape measure for spout reach, height and hole spacing
- A torch to see under the basin or behind the bath panel
- Your phone camera, to photograph the existing connections before you take anything apart
- A towel and bowl, as tails hold water even with the supply off
Measure in millimetres throughout, as tap specifications are given in metric even when the threads have inch names. Our guide to reading a tape measure explains the millimetre scale.
How do you measure a tap for a replacement?
Identify the type of tap first, then measure the holes, the connections and the spout. Work through these steps.
- Count the tap holes. One hole means a monobloc mixer. Two holes mean pillar taps or a two-hole mixer. Three holes on a basin usually mean a separate spout with two handles.
- Measure the hole diameter. Measure across the hole in the sink or bath, or across the base of the old tap’s shank if it is still fitted.
- Measure the hole centres. For two-hole fittings, measure from the centre of one hole to the centre of the other.
- Measure the deck thickness. Measure how thick the basin, sink or worktop is where the tap fixes. Thick stone worktops may need a tap with a longer shank.
- Measure the tail thread. Turn the water off, look under the basin and measure across the threaded part of each tail with a caliper.
- Measure the spout. Measure reach from the centre of the tap hole to where water leaves the spout, and height from the deck to the underside of the outlet.
- Note your water system. Find out whether your hot water is from a combi boiler or unvented cylinder (mains pressure) or from a tank in the loft (low pressure).
What size hole does a mixer tap need?
Most monobloc taps are made for a single hole of around 35 mm, but always check the installation sheet for the tap’s minimum and maximum hole size. Tap hole cutters for stainless steel sinks are commonly sold in a 35 mm size for this reason. A hole that is too large can let the tap twist, while one that is too small simply will not take the shank.
Two-hole bath mixers are a different case. Mira describes its deck-mounted bath taps as having pipe centres 180 mm apart to suit a standard bath with two pre-drilled holes, so measure your hole centres before ordering a bath mixer. Some mixers have adjustable centres to cope with small differences.
How do monobloc tap tails connect?
Monobloc taps use flexible or thin copper tails that screw into the underside of the tap, usually with an M10 metric thread, and connect to 15 mm pipe or a ½ inch fitting at the other end. M8 and M12 tails also exist, so check the thread before buying replacements; M10 means a 10 mm thread diameter. Tails come in lengths such as 300 mm, and the tap usually arrives with its own.
Pillar taps work the other way round: the threaded tail is part of the tap and a tap connector joins it to the pipe. A 15 mm × ½ inch connector suits basin and sink pillar taps, and a 22 mm × ¾ inch connector suits bath pillar taps. If you need to check the pipe size first, our guide on how to measure pipe size covers it, and 15 mm vs 22 mm copper pipe explains why bath feeds are often the larger size.
How do you measure spout reach and height?
Reach is the horizontal distance from the centre of the tap hole to the spout outlet; height is the vertical gap from the deck to the outlet. On a basin, the water should land near the waste, not on the back edge of the bowl, so compare the reach with the distance from the tap hole to the centre of the plughole. On a kitchen sink, check that a tall spout clears any window board or shelf and that the outlet height lets you fill your largest pan.
If you are also replacing the waste, the sink drain size guide explains how plugholes and wastes are measured.
How do you measure three-hole and wall-mounted taps?
For a three-hole basin set, measure the centres of the two outer holes; for a wall-mounted tap, measure the outlet height above the basin and its projection from the wall. Three-hole mixers have a central spout and two separate handles linked by hoses under the deck, and each set is made for a particular spread of holes. Wall-mounted taps are fed by pipework buried in the wall, so a replacement usually has to match the existing concealed valve or fixing centres unless the wall is opened up.
Does water pressure affect which tap you can buy?
Yes. Many modern mixers need a minimum water pressure, so check the specification against your system. Homes with a loft tank feeding the hot water often have low pressure, and some taps are designed specifically for low-pressure systems. The minimum working pressure, usually given in bar, is listed on the tap’s specification sheet. Taps should also comply with the Water Supply (Water Fittings) Regulations 1999, and many manufacturers show this with WRAS approval.
Common mistakes to avoid
- Treating ½ inch as 12.7 mm. A ½ inch BSP tail measures about 21 mm across the thread.
- Measuring the tap body instead of the thread. The shank above the thread or the back nut is wider than the thread itself.
- Ignoring deck thickness. A tap made for a thin steel sink may not reach through a 30 mm or thicker stone worktop.
- Buying a mains-pressure tap for a gravity system. It may only trickle. Check the minimum pressure first.
- Forgetting the isolating valves. Fit or check service valves on each supply so the tap can be maintained later.
Frequently asked questions
Are UK bathroom taps a standard size?
Pillar taps follow standard thread sizes: ½ inch BSP for basins and ¾ inch BSP for baths. Mixer taps vary more, so check the hole size, hole centres and deck thickness against the product details.
How do I know if my tap is ½ or ¾ inch?
Measure across the thread of the tail. About 21 mm means ½ inch BSP, and about 26 mm means ¾ inch BSP. Basin taps are almost always ½ inch and bath pillar taps ¾ inch.
What size are monobloc tap tails in the UK?
Most screw into the tap with an M10 thread and connect to 15 mm pipe or a ½ inch fitting at the other end. Some taps use M8 or M12 threads, so check before buying replacement tails.
What size hole do I need for a kitchen tap?
Most single-hole kitchen taps are designed for a hole of around 35 mm. The tap’s installation sheet gives the exact range it accepts.
How far apart are bath tap holes?
Two-hole baths are commonly drilled for taps with 180 mm centres, which is the spacing Mira gives for its deck-mounted bath taps. Measure centre to centre before ordering, as some baths differ.
Sources
- A guide to bathroom taps, Mira Showers
- ISO 228-1: Pipe threads where pressure-tight joints are not made on the threads, ISO
- The Water Supply (Water Fittings) Regulations 1999, legislation.gov.uk