About 6mm — the width of a pencil
An adult house mouse can get through a hole around 6mm wide. The rule of thumb pest controllers use is simple: if you can push a standard pencil into a gap, a mouse can use it. Young mice manage even less.
Why such a small gap is enough
A mouse's body is soft and flexible, and its ribs are hinged in a way that lets them fold. The limiting factor is the skull. If the head fits, the rest of the animal follows. Whiskers help it judge the width in the dark before it commits.
Where 6mm gaps hide in a Brent home
- Around pipes where they come through walls and floors — often left unsealed after plumbing work
- Behind and under kitchen units, where the service void is rarely finished — see stopping mice behind kitchen units
- Under skirting boards that have shrunk away from old floorboards
- Worn door thresholds and the bottom of garage doors
- Air bricks with large holes or broken grilles — covered in air bricks
- Where a newer extension meets the original wall
In the Victorian terraces of Willesden and Kilburn, with suspended timber floors and decades of alterations, there can be dozens of gaps this size in a single kitchen.
The pencil test
Walk round the rooms where you have seen signs of mice with a pencil and a torch. Try every gap along skirting boards, around pipes and at the back of cupboards. Mark any the pencil enters. That list, and the signs near each one, tell you which to prioritise — how to tell which holes are in use explains the next step.
Why it is not worth filling them straight away
If mice are already living inside, sealing their routes can trap them in wall voids and under floors. Deal with the mice first, then close the gaps — before or after treatment sets out the order.
Mice or rats?
A gap that is too small for a rat may still let in mice, and droppings are the easiest way to tell which you have — see rats or mice. For the rat equivalent, read what size gap a rat can get through.
Getting the gaps closed
Mouse control in Brent is £190 for a flat and £240 for a house. Once activity has stopped, our rodent proofing seals small holes for £180 for up to one hour — book rodent proofing here.
