Practical advice
Nest box hole sizes for UK garden birds
A clear guide to 25mm, 28mm, 32mm and 45mm nest-box entrances, plus open-front boxes for robins.
Measure across the widest part of a circular entrance.
Do not alter a box while birds are using or investigating it.
Avoid perches beneath the hole; nesting birds do not need them.
The entrance hole is one of the quickest ways to rule a nest box in or out for a particular bird. A hole that is too small physically prevents access. One that is unnecessarily large admits more competitors and may leave the nest chamber more exposed.
The familiar measurements below come from current RSPB nest-box guidance. They are useful starting points for UK gardens, but they do not guarantee which species will move in.
Nest-box entrance sizes at a glance
| Hole or entrance | Birds commonly associated with it |
|---|---|
| 25mm round hole | Blue tit, coal tit and marsh tit |
| 28mm round hole | Great tit and tree sparrow; smaller tits can also enter |
| 32mm round hole | House sparrow; also accessible to several smaller hole nesters |
| 45mm round hole | Starling |
| Open front | Robin, wren and pied wagtail |
These measurements apply to purpose-built boxes with an appropriate interior. Drilling the right hole in a tiny decorative box does not make the box suitable.
25mm: the blue-tit option
A 25mm circular entrance is commonly used when the aim is to favour blue tits, coal tits or marsh tits. It excludes great tits and house sparrows, reducing some competition without excluding the smaller target birds.
For a ready-made example, see our guide to the best nest boxes for blue tits. A protected metal or slate entrance edge may reduce enlargement by squirrels or woodpeckers, but it cannot make a poorly positioned box safe.
28mm: room for great tits and tree sparrows
At 28mm, great tits and tree sparrows can enter alongside smaller tits. This can be useful where you want to serve a broader group rather than target blue tits alone.
Do not enlarge a 25mm hole simply because no bird used the box in its first season. Occupation depends on habitat, territory, orientation, temperature and nearby nest sites as well as entrance size.
32mm: the house-sparrow size
A 32mm entrance is the standard reference for house sparrows. Because sparrows often nest near one another, this size is frequently used in multi-chamber terraces. Each hole should lead to its own enclosed compartment.
Our comparison of sparrow nest boxes explains the practical differences between three-chamber designs. Measure retailer specifications carefully: a product name is not proof of its entrance diameter.
45mm: a starling box
Starlings need a much larger entrance and a more generously proportioned box. A 45mm hole in an ordinary tit box does not create enough usable space. Choose a design intended for starlings, with the correct floor area, depth and robust mounting.
The larger opening increases exposure, which makes wall thickness, roof overhang and sheltered positioning particularly important.
Open-front boxes for robins
Robins do not need a circular entrance. They prefer an open-front box or sheltered nesting shelf tucked into vegetation, an outbuilding or another quiet recess. Wrens and pied wagtails may also use this format.
An open front makes concealment essential. Do not display a robin box on a bare fence and expect the roof alone to protect it. Our robin nest-box guide covers suitable ready-made designs and placement.
How to measure a nest-box hole
Measure the internal diameter straight across the centre of a round hole, from edge to edge. A millimetre or two matters, so use a ruler or callipers rather than estimating. For an oval swift entrance, both height and width are relevant and should follow the manufacturer's tested design.
If a wooden entrance has been enlarged by wear or predators, assess it only after the breeding season when the box is definitely unused. Fit a correctly sized plate flush to sound timber. Never trap a bird behind a new plate or narrow an entrance around an active nest.
Hole size is only one part of a safe box
The BTO recommends wooden walls at least 15mm thick, plus drainage, ventilation and access for cleaning. The interior must be deep enough to keep the nest below the entrance and should offer a rough surface that fledglings can climb.
Avoid decorative perches. Hole-nesting birds can grip the entrance edge without one, while a perch may help a predator reach inside. Check our best nest boxes for the full buying checklist.
Never resize an occupied box
Wild birds, eggs and nests are protected. Do not drill, file, fit a plate or open a box once birds are building or nesting. If you are unsure whether a box is in use, leave it undisturbed and watch quietly from a distance.
Make repairs during the lawful maintenance period only after you have established that the box is empty. The timing and method are covered in how and when to clean a nest box.