Mice, and the 6mm gap that is doing all the work
A house mouse gets through a gap about the width of a pencil. Until that gap is shut, every mouse you kill is a vacancy, and the bait is a subscription rather than a fix.
By The Pest GuyLast updated
The short answer
This is a proofing job with some trapping attached, not the other way round.
Mice get in through a gap of about 6mm. Three separate council pest control services say so in almost the same words, and it is the only number on this page you need to act on. Nothing else about a mouse problem changes until that gap is found and shut.
Which is why the usual order of work is the wrong way round. Bait and traps deal with the mice that are already inside. They do nothing at all about the reason there are mice inside, so the population comes back on the same route it used the first time.
Stroud District Council says this about its own paid treatment, in writing: "Whilst our treatment can remove rodents present at the time we cannot guarantee that they will not return in the future." That is a council pest control service telling you that the treatment is the half of the job that does not last.
How small the gap actually is
Oxford City Council: "Even gaps as small as 6mm, like those around pipes, can allow entry, so sealing these is crucial." Stroud District Council: "Mice can enter through gaps larger than 6mm." Rochdale Borough Council skips the tape measure and gives you the version you can do standing up, which is that mice only need a gap the diameter of a pencil.
Six millimetres is smaller than most people picture when they hear the word gap. It is not a hole. It is the daylight under a back door that has dropped on its hinges, the ring of space round a waste pipe where it goes through the wall, a cracked air brick, a perished seal on a cellar hatch. None of that reads as damage. All of it reads as a door to a mouse.
Stroud names the places to start: broken air bricks, badly fitted or rotted doors, holes around any pipes. Rochdale adds the two that get missed, which are gaps around heating and water pipes where they pass through a floor or wall, and ventilation that has lost its mesh. Rochdale specifies 3.15mm mesh for ventilation, which is the point: you are replacing airflow, not blocking it.
Do this bit with a torch, at night, from inside, with the lights off in the room you are looking into. Daylight through a gap is easier to see than the gap is.
One stop rule before you start filling anything. If you are working in a loft, in eaves, or under a roofline and there is any sign of bats or of a bird nesting, stop and take advice before you touch it. Bats and their roosts are protected, and so are wild birds and their active nests. That is not a technicality you can decide about yourself.
Telling mice from rats before you buy anything
Worth two minutes, because the answers diverge afterwards. The droppings are the fastest tell. Rochdale describes mouse droppings as black, 3 to 6mm, similar to a grain of rice, scattered rather than piled. The Mammal Society describes brown rat droppings as putty-like, with a blunt end and a pointed end, up to 17mm long and 6mm wide. Those are not two versions of the same thing.
Rochdale adds the detail that explains why droppings are usually the first thing anyone finds: each mouse can leave up to 80 of them a day, most often under kitchen sinks and units, around central heating boilers, and in roof spaces.
Size is the other one. An adult mouse is 60 to 90mm of body with a tail of about 100mm, and weighs under 25 grams. A brown rat is 150 to 270mm of body and usually 200 to 300 grams. If you have seen the animal and argued with yourself about which it was, it was probably a mouse.
After that, the signs are the same family: smear marks, which Rochdale describes as dark grey or black marks left on surfaces where the animal runs the same route against a wall, and gnawing on wood, carpet, paper, pipe, cable and furniture. Mice also, as Oxford City Council puts it, cannot control their bladder, which is why the contamination is spread along the run rather than left in one place.
What this page cannot do is tell you which animal you have. Nobody can do that from a description, and this site does not pretend otherwise. What the two lists above do is let you decide for yourself, which is the part a page selling a callout has no reason to hand you.
Why a fortnight of doing nothing matters
The Mammal Society records house mouse litters of between five and eight, "born in roughly monthly intervals in buildings". Outside a building, females produce young only in the warmer months. Inside one, the seasonal brake comes off. Young are born blind and weighing as little as 1.5 grams, and are fully weaned by 23 days.
That is the honest answer to how many mice you have, and it is a rate rather than a number. There is no reliable way to count them from what you can see, and the number a page quotes you is invented. The rate is the thing that should make you move: a warm building takes the season out of the equation, so a fortnight of meaning to get round to it is a real amount of time.
Two consequences, both sourced, and neither of them scaremongering. Rochdale lists salmonella and listeria among the diseases mice spread, through contaminated food and surfaces. And mice gnaw continuously to keep their incisors down, on wood, carpet, paper, pipe and cable. Oxford City Council names what that means where it hits wiring: a fire risk from damaged cables.
Neither of those is a reason to panic and both are reasons not to leave it. The cable one in particular is why living alongside a couple of mice is a worse position than it sounds.
What you are allowed to use, and the order to do it in
Start with the one that catches people out, because it is still on shelves. Setting a glue trap for a rodent is now a criminal offence for a member of the public across Great Britain, on three different dates. In England the Glue Traps (Offences) Act 2022 says "a person who sets a glue trap in England for the purpose of catching a rodent commits an offence", and it came into force on 31 July 2024, with a licensing route for professionals only. In Wales the ban has run since 17 October 2023 under the Agriculture (Wales) Act 2023. In Scotland, since 1 July 2026, it is an offence to use, supply or possess one at all, with a stated maximum of a £40,000 fine or twelve months in prison.
You can still buy them. That is the trap, in both senses. If you have some in a cupboard, the answer is not to set them.
On poison, GOV.UK is short and worth reading before you buy anything: only use poison to kill the pests it is intended for, and that will be written on the packaging. Traps must be approved for use with the species you want to control. And you have to protect everything you were not aiming at, by placing lethal traps under cover so other animals and birds are not caught, and by preventing wildlife from eating poison you have put down.
There is a further layer that shop shelves do not explain. HSE says rodenticide products sold to the general public may be restricted in how much active substance they contain, in pack size, and in where they can be used, for example only within a bait box. Professional products are restricted in pack size and in who they can be sold to, and professional users of anticoagulant rodenticides in Great Britain and Northern Ireland have to work within a stewardship regime that involves certification and proof of competence at the point of sale. Underneath all of it sits one line of the biocides regulation: biocidal products are to be used in compliance with the terms and conditions of the authorisation. The label is not advice.
So this page names no product and tells you to buy nothing. Here is the order instead. Find the gap, at night, with a torch. Shut it properly, with something a mouse cannot chew back through, and mesh rather than filler wherever the hole is there for air. Move the food, which means dry goods and pet food into sealed containers rather than bags. Then, and only then, deal with the animals still inside.
If the building will not give up its gap, that is the honest point to pay someone, because you are buying a survey of the fabric rather than a box of bait. Ask what they found and how they intend to close it. A quote that is all treatment and no proofing is a quote for the mice you can see, not for the way in.
About this resource
The 6mm figure, the proofing points, the droppings sizes, the smear marks and the disease and cable-gnawing risks come from the pest control pages of Oxford City Council, Rochdale Borough Council and Stroud District Council. The house mouse sizes and breeding rate come from the Mammal Society species record. The glue trap position comes from the Glue Traps (Offences) Act 2022, the Agriculture (Wales) Act 2023 and the Scottish Government's announcement of the Scottish ban. The rules on permitted methods come from GOV.UK, and the position on who may buy which rodenticide from HSE and from Article 17 of the biocidal products regulation as retained in Great Britain.
Two honest limits. This page names no product, no rodenticide and no company, because the restrictions on who may buy what are real and a recommendation is not this site's to make. And it gives you no figure for how many mice are in your house, because that figure does not exist. This is not legal advice: where an Act is named here, check it against your own situation before you rely on it.
Common questions
How many mice are there if I see one?
Nobody can tell you, and anyone who gives you a number is guessing. What is known is the rate rather than the count: the Mammal Society records house mouse litters of five to eight born at roughly monthly intervals in buildings, with young fully weaned by 23 days. So the useful question is not how many there are today. It is how long the gap has been open and how long it stays open.
How are mice getting in my house?
Through something around 6mm wide. Oxford City Council puts it as "even gaps as small as 6mm, like those around pipes, can allow entry", and Rochdale Borough Council uses the same test in a form you can do without a tape measure: mice only need a gap the diameter of a pencil. Stroud District Council lists where to look first, which is broken air bricks, badly fitted or rotted doors, and holes around any pipes.
Is it mice or rats in my house?
The droppings settle it and it is not close. Mouse droppings are 3 to 6mm, black, and Rochdale describes them as similar to a grain of rice. Brown rat droppings, on the Mammal Society record, are putty-like with one blunt end and one pointed end, and run up to 17mm long and 6mm wide. Body size is the other tell: an adult mouse is 60 to 90mm of body, a brown rat is 150 to 270mm.
Should I be worried if I have mice in my house?
Worried is the wrong word. Two things are worth taking seriously and both are sourced. Rochdale lists salmonella and listeria as diseases mice spread. And mice gnaw constantly to keep their teeth down, which Oxford City Council links directly to a fire risk from damaged cables. That is the real argument for dealing with it properly rather than living alongside them.
Will the council get rid of mice in my house?
Possibly, and probably not for nothing. GOV.UK says to contact your council to find out if they provide pest control services and that you may be charged a fee. Direct Line Home Insurance put a Freedom of Information request to every UK and Northern Irish local authority in January 2025 and got 270 usable replies: only 5 per cent treat rats and mice free of charge, the rest charge between £10 and £283, and 24 per cent of councils no longer offer a pest control service at all.
Do landlords legally have to get rid of mice?
Often yes, and the route depends on why the mice are there. If they are getting in through something the landlord has to repair, section 11 of the Landlord and Tenant Act 1985 covers the structure and exterior. Separately, pests are one of the 29 prescribed hazards behind the fitness for human habitation covenant. Which one applies, and what a tenant can actually do about it, is the whole of the resource on who is responsible for pest control.