Installer measuring a ceramic basin tap hole

Tap Hole Sizes: Basin, Kitchen and Thread Drill Sizes

The standard tap hole in a UK basin or kitchen sink is about 35mm in diameter. That is the size most single-hole (monobloc) taps are made for, though some need 32mm and two-hole taps often fit smaller holes. “Tap hole size” also means something else to engineers: the drill size you use before cutting a thread with a tap. This guide covers both, so you can measure the right thing and buy once.

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TL;DR

  • Basin and kitchen taps: the usual hole is 35mm. Some monobloc taps need 32mm, and two-hole taps may fit holes of 25-35mm, depending on make and model.
  • Hole count matters as much as diameter: one, two or three holes, each with its own spacing. Count the holes before you look at any tap.
  • Check the tap’s own specification before you buy, and measure the hole itself with a tape or calipers rather than guessing.
  • Thread tapping is different: the drill is smaller than the bolt. An M5 coarse thread needs a 4.2mm drill, and a 1/4-20 UNC thread needs a #7 drill (5.11mm).
  • Waste fittings have their own sizes, so check them separately. Our guide to basin waste size covers that.

Table of Contents

Which kind of tap hole size do you mean?

Two different jobs share this phrase.

1. The hole in the sanitaryware or worktop. This is the hole in a basin, sink or worktop that the tap’s tail passes through. You need it when replacing a tap, choosing a new basin or planning a bathroom refit.

2. The drill hole for a thread. When you cut an internal thread in metal with a tap, you first drill a “tap drill” hole. It has to be smaller than the bolt, because the thread is cut into the walls of that hole. Engineers, fabricators and maintenance teams search for this.

The first half of this article is about plumbing, the second about threads. If you maintain washrooms for a living you will probably need both, because fixing a dispenser, towel bar or hook to a metal partition often means tapping a hole.

Which kind of tap hole size do you mean? — overview diagram

Standard tap hole sizes for basins and sinks

The 35mm rule of thumb

Guides to UK bathroom fittings agree on the headline figure. Basin taps are typically designed for holes of about 35mm, and the same size turns up for kitchens: the standard kitchen sink tap hole is 35mm, the size of the hole in the sink or the cut-out in a worktop.

Two cautions apply:

  • It is a typical size, not a rule. It varies with the type of tap and the manufacturer.
  • The tap’s base is always bigger than the hole. Kitchen tap bases usually run from 44 to 60mm so they cover the hole neatly. So the visible footprint of a tap tells you nothing about the hole behind it.

Where 32mm and 25mm appear

Single-lever monobloc taps usually want 35mm, although some only need 32mm. Two-hole pillar or deck-mounted taps take one feed per hole, hot or cold, so their holes can be smaller. The same source puts them at 25-35mm depending on the model.

The practical point: a 35mm hole is fine for a tap that needs 32mm only if the tap’s tail and fixing nut are made to seat cleanly. A hole that is too big leaves the tap loose and the seal weak. A hole that is too small will not take the tail at all. Always read the manufacturer’s installation sheet for the hole range the tap accepts.

A quick reference table

Fitting Typical hole What to check
Single-hole basin mixer (monobloc) About 35mm Exact tail diameter in the tap’s spec sheet
Single-hole kitchen mixer 35mm, sometimes 32mm Worktop thickness and base size (44-60mm)
Two-hole pillar or deck taps 25-35mm per hole Spacing between the holes
Three-hole basin set Varies by design Spacing and which hole takes the spout
Wall-mounted taps No hole in the basin Pipework position and spout reach

The diameters in the first four rows come from the guides linked above and a further summary of basin tap hole sizes, which also gives the metric figure as roughly 1 3/8 inches. Spacing and spout reach are set by each design, so we have not put numbers on them here. Use the product’s own data.

Tap hole configurations: one, two or three holes

The number of holes in the basin decides which taps you can fit, so settle this before anything else.

One hole. A single mixer tap with one lever, or one tap body carrying both controls. Modern basins are usually drilled this way and a monobloc suits them.

Two holes. Separate hot and cold taps, often called pillar taps, one in each hole. The spacing between the holes varies by basin design. If you want a single mixer on a two-hole basin, you need a fitting made for it, or a blanking plug in the spare hole.

Three holes. A spout in the middle with a hot and a cold control either side. These are common on traditional sets. Basin sets in this layout have spacing that follows each design, so you cannot swap in a different brand and expect it to line up.

No holes. Some basins are sold with no tap hole at all, because the tap is mounted on the wall or the worktop. Here the question becomes pipework position, not hole size.

Four basin tap hole configurations

If you are changing the layout, say from three holes to one, you generally cannot make the hole bigger in ceramic yourself without a real risk of cracking the basin. Buy a basin drilled for the tap you want, or choose a tap that fits the holes you already have.

How to measure a tap hole properly

  1. Remove the old tap if you are replacing one, or look under the basin or sink if it is fitted. Isolate the water first.
  2. Count the holes in the basin or worktop.
  3. Measure the diameter of each hole across the widest point, in millimetres. Calipers are best, a steel rule will do.
  4. Measure the centre-to-centre distance between holes if there is more than one.
  5. Measure the thickness of the basin or worktop at the hole. Thick stone or solid-surface worktops can exceed the length of a tap’s tail and fixing.
  6. Check the overhang and clearance behind the tap: a lever or spout that hits a splashback or window sill is a common surprise.
  7. Compare with the tap’s spec sheet, which gives the hole size it accepts and the thickness it can clamp.

One warning from a user report quoted by the LODECE guide: a homeowner found their hole was roughly 30mm, too small for the tap thread to seal properly. Measure first, then buy.

Perspective: what we see when customers get this wrong

We sell bathroom and washroom products online to homeowners, contractors and facilities teams, and the same pattern keeps coming up. Buyers fall for a tap or basin on looks, then discover the fittings do not match what is already in the wall or on the floor. The cost is not the item itself but the delay: a second delivery, a plumber booked twice, a washroom out of use.

Our advice is simple. Settle the connection sizes before you settle the style. Hole count and diameter, waste size, and pipework position are boring, and they decide whether the job takes an afternoon or a fortnight. Our guide to basin waste size and UK limits is a good example of the second check, and it is worth reading alongside this one.

For commercial buyers, specify one standard across a whole site wherever you can. Matching hole sizes and fittings across fifty washrooms means one spare part to stock, not five.

— Carl

Tap drill sizes for threads

Now the engineering meaning. When you cut an internal thread, the hole you drill first is not the bolt size. The reason is that the tap cuts the thread into the sides of the hole, so the hole must be smaller than the bolt’s outside diameter.

The metric rule

For metric coarse threads, the tap drill is the major diameter minus the pitch. So an M6 x 1.0 thread uses a 5.0mm drill. An M8 at 1.25mm pitch uses 6.75mm.

Common sizes at roughly 75% thread engagement:

Thread Coarse pitch Tap drill Clearance drill (free fit)
M3 0.5mm 2.5mm n/a here
M5 0.8mm 4.2mm 5.5mm
M6 1.0mm 5.0mm (formula) n/a here
M20 2.5mm 17.5mm n/a here

What size hole for a 5mm tap?

For a standard M5 coarse tap, drill 4.2mm. That is the answer to the most common version of this question. If you instead want a hole for an M5 bolt to pass through freely, that is a clearance hole, and 5.5mm is the figure given for M5.

Imperial (UNC and UNF) threads

For 1/4-20 UNC, the standard tap drill is a #7 drill, 0.201 inches or 5.11mm, for 75% engagement. The free-fit clearance drill on the same page is 17/64 inch. A common mistake is to drill 0.250 inch, the major diameter, which leads to a failed thread. Always confirm the threads per inch too: 1/4-28 UNF needs a #3 drill, 0.213 inches.

What size drill for a 3/8 inch hole?

There is no single answer, because it depends on the job. For a 3/8-16 UNC thread, the tap drill is 5/16 inch (7.94mm). For a clearance hole where a 3/8 inch bolt passes through freely, 25/64 inch (9.92mm) is typical.

Very small and form taps

For a 000-120 tap, the recommended tap drill is a #71 bit. A form tap, which displaces metal instead of cutting it, needs a different hole. For a 1/4-20 form tap, some manufacturers recommend 5.8mm to 5.9mm depending on the material, larger than the cutting tap drill. Follow the tap maker’s chart for form taps.

When to drill slightly larger

The 75% figure is a guideline. In tough alloys such as stainless steel, machinists often go up a little, lowering engagement to around 65-70%, which cuts the chance of snapping the tap. The trade-off is a slightly weaker thread. For fixing a bracket, that is almost always worth it. For a load-bearing joint, stay with the standard drill and take it slowly with cutting fluid.

Putting it together: a washroom example

Imagine a facilities manager refitting three washrooms. The job has several hole sizes in it, and none of them are interchangeable:

  • Basin taps: count holes in each basin, then confirm the diameter, typically around 35mm, against the tap’s specification.
  • Waste: check the waste outlet and the waste fitting match. See our basin waste size guide.
  • Fixings for accessories: if a towel bar or hook goes onto a metal partition or frame rather than a wall plug, you may be tapping an M5 thread, which means a 4.2mm pilot hole.
  • Spare holes: if you move from a three-hole to a one-hole tap layout, plan blanking plugs or a new basin. Do not enlarge ceramic.

A short checklist on the first site visit saves a repeat trip.

Washroom and bathroom products from Simco Direct

If you are specifying a refit, we supply washroom accessories, heating and bathroom products for homes and commercial sites, with bulk order pricing for trade buyers, fast UK delivery and a 30-day returns policy. Browse the range on the Simco Direct homepage, or look at fittings such as the Bobrick 19mm Towel Bar and the Bobrick Utility Hook, which are the sort of items you fix to a partition or wall. For the hand dryer side of a washroom, the Bobrick TrimLine Automatic Hand Dryer in white is one to look at.

Mistakes to avoid

  • Buying the tap before measuring the hole. Diameter and hole count come first.
  • Trusting the base of the tap. The visible base, often 44-60mm on kitchen taps, says nothing about the hole size.
  • Drilling a thread hole at bolt size. The tap drill is smaller than the bolt.
  • Mixing up tap drill and clearance drill. One is for the threaded part, one for the part the bolt passes through.
  • Enlarging a ceramic basin hole by hand. The basin can crack. Choose a different tap or basin.
  • Forgetting thread pitch. 1/4-20 and 1/4-28 look the same and need different drills.

FAQ

What is the standard tap hole size in the UK?

About 35mm for most single-hole basin and kitchen taps. Some monobloc taps need 32mm, and two-hole taps can fit holes of 25-35mm. Check the specification of the exact tap, as the figure varies by manufacturer.

Can I fit a different tap into an existing hole?

Only if the new tap’s tail suits the hole diameter and the number of holes matches. A single mixer on a basin with one 35mm hole is usually straightforward. Moving from a one-hole to a three-hole tap, or the reverse, normally means a different basin or blanking plugs. Do not try to enlarge ceramic yourself.

What drill size do I use to tap an M5 thread?

Use a 4.2mm drill for a standard M5 x 0.8 coarse thread, which gives roughly 75% thread engagement. For an M5 bolt to pass through freely, drill a 5.5mm clearance hole instead.

Why is the drill smaller than the bolt when tapping?

The tap cuts the thread into the wall of the hole. If the hole were as big as the bolt, there would be no metal left to cut. For metric coarse threads, subtract the pitch from the major diameter to find the drill size.

Should I use a bigger drill for stainless steel?

Many machinists do, lowering thread engagement to around 65-70% to reduce tap breakage. The thread is a little weaker, so keep the standard size for joints that carry heavy loads.

Sources

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