At 28 mm, tit nest box entrances keep larger birds out

A 28 mm entrance suits great tits while helping to keep out many larger garden birds. If you want blue tits or coal tits, use a smaller 25 mm hole; a 32 mm opening is less selective and may admit larger species.

For reliable tit nest box dimensions, use a 120 × 120 mm internal floor, a 200 mm front, a 250 mm back, and position the entrance hole 125–150 mm above the floor.

Choose the right hole size first

Choose the smallest entrance diameter that suits the species you want.

Match the hole to the bird

Treat 32 mm as a wider door

A 32 mm entrance can suit great tits, but it also admits house sparrows and other larger potential occupants.

Build safe internal dimensions

Build around the internal space, not the outside measurements.

Use a 120 × 120 mm internal floor, a 200 mm front, and a 250 mm back, with the lower edge of the hole 125 to 150 mm above the floor.

Cut timber from the inside out

Use untreated timber 15 to 18 mm thick, and check the internal dimensions before buying a ready-made box.

Add the features chicks need

Add drainage holes, high ventilation holes and a rough inner face below the entrance so fledglings can climb out.

For a simple cut plan, use 18 mm untreated timber and keep the finished internal floor at 120 × 120 mm. Cut one 120 × 120 mm floor, two side panels that rise from 200 mm at the front to 250 mm at the back, a 200 mm front and a 250 mm back. If the front and back panels sit over the side edges, make them 156 mm wide to preserve the 120 mm internal width. A 196 × 216 mm roof gives about a 50 mm front overhang, a 10 mm rear overhang and 20 mm at each side.

This bird box internal dimensions plan makes the nest box floor size, exterior dimensions and tit box roof overhang clear before cutting. For a 28 mm hole, mark its centre 149 mm above the inside floor, giving a 135 mm entrance depth below the lower edge.

Small construction details help a garden bird nesting box stay dry without becoming draughty. Drill four 5–6 mm bird box drainage holes near the corners of the floor, rather than one large central hole that can clog with nesting material. Add two 5–6 mm nest box ventilation holes high on each side, immediately below the roof line, so air can escape above the nest cup. Use a roof that projects at least 40–50 mm beyond the front and around 20 mm beyond the sides; this keeps much of the rain away from the entrance and walls.

Leave the inside of an untreated timber bird box rough, or score a shallow climbing texture beneath the hole, but do not create deep grooves where dirt or parasites can collect.

Check or adapt a ready-made box

Check the floor, depth, drainage and timber before paying for a ready-made box.

Compare materials before buying

Reduce an oversized entrance safely

Fit a flat, weather-resistant reducer plate over an oversized hole, ensuring it is secure, smooth and cannot trap water.

To reduce a 32 mm entrance to 28 mm or 25 mm, fit a solid reducer plate only when the box is empty and structurally sound. A practical plate is a square of durable untreated hardwood or exterior-grade plywood, roughly 100 × 100 mm and 9–12 mm thick, with the new hole drilled cleanly in its centre. Hold it flat against the outside of the front panel, make sure its lower edge does not obstruct the original opening, and secure it with four short, countersunk screws positioned well away from the hole.

Seal neither the entrance nor the plate edges with expanding foam or wet adhesives. Check that there are no splinters, protruding screws or gaps that could admit rain, and confirm that the added plate does not make the entrance tunnel unusually deep.

Place the box and avoid nesting-season harm

Place the box 2 to 4 metres above ground with a clear flight path.

Face it north-east to east where practical, fix it firmly and give it a slight forward tilt so rain runs off the roof.

Keep disturbance low

Put boxes up in autumn or late winter, avoid close checks during nesting, and clean empty boxes only in autumn with a dry brush.

Know when this box is wrong

These dimensions suit small-hole-nesting tits, not open-fronted species such as robins, wrens or pied wagtails.

Your questions answered

What size hole should I use for a tit box?

Use 25 mm for blue tits and coal tits, or 28 mm for great tits. Choose 32 mm only if house sparrows are acceptable.

How big should a blue tit nest box be?

A blue tit box needs a 120 × 120 mm internal floor, a 25 mm entrance and 125 to 150 mm below the hole.

Can great tits fit through a 25 mm hole?

Great tits usually need a 28 mm entrance for reliable access; 25 mm is intended for blue tits and coal tits.

Should a tit nest box have a perch?

No. Tits cling to the front, while an external perch can help predators reach the entrance.

Which way should a blue tit box face?

Face it north-east to east in a sheltered place, avoiding strong midday sun, driving rain and blocked access.

When should I clean out a bird box?

Clean an empty box in autumn after breeding, brushing out old nesting material without using chemicals.

Choose your box and put it up

Choose the smaller suitable entrance, then prioritise a stable, dry box with correct internal dimensions.

Buy or build a box with a 120 × 120 mm internal floor, drainage holes, a projecting roof and a 25 mm or 28 mm entrance for your target tit.

Editorial Team

Our team consists of passionate British birdwatchers and garden wildlife enthusiasts. We provide honest, practical advice on feeding garden birds, choosing the right feeders, and making your outdoor space a haven for UK wildlife — guided by RSPB best practice and real garden experience.