Downlight Cut-Out Sizes Explained

A downlight's cut-out size is the diameter of the hole you need in the ceiling, not the size of the fitting you can see. In the UK, most domestic downlights use a cut-out between 50mm and 100mm, with mini fittings going down to 25mm and 65mm to 75mm covering the large majority of ceilings. Before you buy anything, measure your existing hole across its widest point and match it to the cut-out figure (or cut-out range) on the fitting's spec.

This guide covers how cut-out size is defined, how to measure it, the standard UK sizes, and what to do when your hole is too big or too small. Any mains wiring work should be carried out or checked by a qualified electrician, and the circuit isolated at the consumer unit before you go near a ceiling hole.

What cut-out size actually means

Every recessed downlight has three diameters, and manufacturers do not always make clear which one they are quoting.

The bezel is always larger than the cut-out, usually by 15mm to 30mm in total, so a typical 65mm cut-out fitting has a bezel of roughly 80mm to 95mm. That overlap hides a slightly ragged edge, which is why a hole a couple of millimetres oversize is rarely a problem.

If a listing quotes a single number without saying which it is, look for the words "cut-out" or "hole size" on the data sheet. Ordering by bezel diameter is one of the most common ways people end up with fittings that will not fit. For the wider picture, see our Ultimate Guide to LED Downlights.

How to measure your existing cut-out accurately

You are measuring the hole in the plasterboard, not the trim you can see.

1. Isolate the circuit at the consumer unit and confirm it is dead before removing anything.
2. Drop the existing fitting out of the ceiling. Most pull straight down against their spring clips. Support the weight rather than letting it swing on the cable.
3. Measure across the bare hole with a steel rule or the depth blade of a vernier caliper, taking the widest point. Plasterboard holes are rarely perfectly round.
4. Measure a second hole at the far end of the room. Installers sometimes change hole saw partway through a job.
5. Note the state of the edge. Crumbly or torn edges reduce what the spring clips have to grip, which changes your options.

A quick sanity check: if you have old GU10 halogen fittings, the hole is very likely 65mm, 70mm or 75mm. Our guide to replacing halogen downlights covers the rest of that swap.

Common UK downlight cut-out sizes

There is no single British standard hole size, but the market has settled on a handful of figures.

As a rule of thumb, if you are specifying a new ceiling from scratch, a cut-out in the 65mm to 75mm band gives you the widest choice of replacements in ten years' time. If your void is shallow, look at low profile downlights, and for accent work mini downlights use much smaller apertures.

Adjustable spring clips and cut-out ranges

Modern downlights are held in by two or three sprung steel clips. You squeeze them up, push the fitting into the hole, and they spring out to pinch the plasterboard from above. That clip travel is what lets a fitting be sold with a range, for example 65mm to 75mm. Three things worth knowing:

• It is a fit range, not a licence to be sloppy. At the top of the range the clips are near the end of their travel, so grip is weaker. Aim for the middle.
• The bezel still has to cover the hole. A fitting rated to 75mm has a bezel wide enough to hide a 75mm hole, which is why the range and the bezel size go together.
• Fire-rated fittings are tested at a stated cut-out. Where one quotes a range, that range is what was tested. Going outside it is a fire performance problem, not just a fitting problem.

Clips also assume plasterboard in the normal 9.5mm to 15mm band, so on a double-boarded ceiling check the fitting's stated ceiling thickness as well as its cut-out.

When the new fitting needs a smaller hole than you have

This is the harder direction, because you cannot un-cut plasterboard. There are four realistic options, in rough order of effort.

1. Choose a fitting with a wider cut-out range or a bigger bezel

Easiest fix by a mile. If your hole is 75mm, buy a fitting rated 65mm to 75mm rather than a fixed 65mm one. Some ranges also offer an oversize bezel covering up to roughly 90mm, letting a small-aperture fitting sit in a bigger hole.

2. Use a converter plate

A converter (or reducer) plate is a thin disc with a smaller hole in the middle. It fixes over the existing hole and the new fitting goes into the plate. Cheap and quick, but check the plate does not stop the fitting's own clips gripping, and never assume one is compatible with a fire-rated fitting, because an untested plate can compromise the fire seal.

3. Patch and re-cut the plasterboard

The proper fix. Cut a rectangle of board out around the hole, screw in a backing batten and patch, fill, sand and re-cut at the size you want. It is the only option that fully restores a fire-rated ceiling, and the right one if the existing edges are torn.

4. Move the fitting

If the layout is changing anyway (see our spacing guide), patching old holes and cutting fresh ones is often no more work.

When the new fitting needs a bigger hole

Enlarging is simple in principle and easy to get wrong in practice, because you are cutting blind into a void containing cables and joists.

Cutting safely

• Isolate the circuit at the consumer unit, and prove dead. Cables run to and around every downlight position.
• Look in the hole first with a torch and feel around above the board, checking for cables, pipework and the edge of a joist.
• Check joist proximity. A hole that grows 5mm or 10mm can suddenly meet a joist, and a hole saw that hits timber will stall or wander.
• Watch for insulation. Loft insulation touching a fitting is a heat problem unless it is rated to be covered.
• Remove the pilot drill from the hole saw, or use a proper downlight cutter. There is nothing left in the middle for a pilot bit to bite into.
• Support the offcut and cut slowly, so the ring of board does not tear the paper face away.

Enlarging a fire-rated cut-out voids the rating

This is the point most people miss. A fire-rated downlight is tested as a system: a specific fitting, in a specific cut-out, in a specific ceiling construction. The intumescent seal is sized to close that exact aperture when it foams up in a fire. Cut oversize and the seal may not bridge the gap, so the ceiling no longer performs as the certificate says.

So cut to the size the fitting states, not "a bit bigger to make it easier", and patch and re-cut any hole that is already too large rather than reaching for a bigger bezel. Our fire-rated downlights guide explains where fire rating is required, and fire-rated downlights is the place to start once you know your size. This is a building regulations matter, so check with a qualified electrician or your building control body if you are unsure.

Hole saw sizing versus stated cut-out

Hole saws come in fixed sizes, so you will rarely find one that exactly matches a stated cut-out. A little oversize is normal, because the bezel covers it.

As a rule of thumb, allow the finished hole to be 1mm or 2mm over the stated cut-out and no more. Beyond that the clips lose grip, and on a fire-rated fitting you are outside its tested aperture. Do not deliberately undersize and force the fitting either, because you will crack the board face. Staying at or just below the middle of an adjustable range has a useful side benefit, in that it leaves a little room to open the hole up later if you change fitting, but grip is the reason to do it. Cut one hole, test-fit the actual fitting, then cut the rest.

Depth and void clearance: the other half of the fit

Cut-out size only answers half the question. The fitting also has to go up into the void, with room around it and above it. As a rule of thumb, allow roughly 45mm to 70mm of depth for a standard integrated fitting, and 80mm to 110mm for a fire-rated GU10, plus space for the clips to swing clear of any joist or duct and the manufacturer's stated gap above the fitting unless it is rated for insulation contact. Depth, driver position and the practical differences between the two fitting types are covered properly in our GU10 vs integrated LED downlights guide.

Shallow ceilings, concrete soffits and rooms under a bathroom floor are the usual culprits. If you are tight on depth, a low profile downlight with a remote driver often solves it without changing the cut-out at all.

Your hole is X, the fitting needs Y

Once you have measured, use this table.

Replacing only some fittings in an existing ceiling

Mixing new and old in one room is where mismatches become visible.

• Match the bezel diameter, not just the cut-out. Two fittings can share a 65mm hole and still look wrong if one bezel is 80mm and the other is 95mm.
• Match the finish and beam angle too. A single fitting in a different white stands out more than a size difference. See our guide to downlight finishes and bezels.
• Match colour temperature. A 4000K fitting in a row of 3000K ones is instantly obvious. See our colour temperature guide.
• Buy a spare. Buy one extra of the same model and put it in the loft, because ranges get discontinued.

On a whole house, buying in one go keeps the batch consistent.

Getting it right first time

Measure the bare hole, not the trim. Match it to the fitting's stated cut-out or range, and check depth and side clearance at the same time. If your hole is too small, enlarging with a hole saw is straightforward, provided you isolate the circuit and check for cables and joists first. If your hole is too large, a wider-range fitting, an oversize bezel or a converter plate will normally cover it, but patch and re-cut whenever the fitting is fire-rated or the board edge is damaged.

Above all, never open a fire-rated fitting's cut-out beyond its tested size to make the job easier. Browse LED downlights by the size you measured, and if you are unsure about the ceiling construction or the wiring, get a qualified electrician to confirm before you cut.