How Small a Gap Can a Mouse Get Through?

The Skull Is the Limit

A mouse has no collarbone in the way a larger animal does and its ribcage compresses considerably, so the widest rigid part of it is the skull. If the head fits, the mouse fits.

In practice that means a gap of roughly 6mm for an adult and less for a juvenile. Juveniles matter because they are the ones exploring, and a route a young mouse finds becomes a route the whole population uses.

How to Test a Gap Yourself

A standard biro is about the right diameter. If you can push a pen into a gap, treat it as a mouse entry point. A pencil is a slightly more generous test and still worth using on anything you are unsure about.

Hold a torch at a shallow angle to the surface rather than straight at it. Gaps that are invisible head-on throw a clear shadow when the light rakes across them, which is how you find the ones behind a pipe or under a unit.

It Is Not Only About the Gap Being Small

Mice also gnaw, so a gap that is currently too small is not permanently too small. Softwood, plastic pipe, expanding foam, plaster and thin render are all things a mouse will enlarge if there is a reason to.

That is why proofing has to be done with something they cannot work on — metal mesh, mortar, or a plate correctly fitted — rather than something that simply fills the space.

And They Climb

Height is no protection. Mice climb brickwork, cables, pipes and the inside of cavity walls without difficulty, and they routinely get in at first-floor level and above.

In a flat, that means a gap around a service riser three floors up is as much of an entry point as anything at ground level. People check the doors and skirting and never look at the boxing in the hall cupboard.

Where the 6mm Gaps Actually Are

Around the waste and supply pipes under the sink. Behind the boiler where the pipework passes through. Where a cooker hood duct goes through the wall. At the meter and telecoms cable entries. Under a garage or side door. In damaged air brick grilles. At the joint where an extension meets the original brickwork.

Almost all of those exist in almost every house. Which is why proofing is a survey and an hour of work rather than a single repair, and why it is priced at £180 for up to an hour rather than per hole.