How small a hole can a mouse fit through?

Published November 4, 2026

The CDC says mice can fit through a hole the width of a pencil, about a quarter inch. The Internet Center for Wildlife Damage Management says rats can get through openings larger than half an inch. Seal gaps that size or bigger with metal, hardware cloth or mesh plus sealant.

The short answer

A mouse can fit through a hole about a quarter inch across. The CDC puts it as a hole the width of a pencil. The Internet Center for Wildlife Damage Management uses the same quarter-inch figure for mice and says rats can get through any opening larger than half an inch.

That surprises most people. A gap you’d never notice from a few feet away is big enough. A mouse’s skull is the widest hard part of its body, and if the head fits, the rest follows.

Test with a pencil

When you walk your house, carry a pencil. If a pencil fits into a gap, a mouse can use it. For rats, think of the width of your thumb.

Where quarter-inch gaps hide

On a typical Waco house, look here first.

Brick homes on slabs

• Weep holes along the bottom row of brick. These are meant to drain water and are often big enough for a mouse.
• The hole where the A/C line set enters the wall. It’s often cut larger than the lines.
• Gaps around plumbing, gas and electrical lines where they enter.
• Garage door corners, where the door seal doesn’t meet the frame.
• Gaps under exterior doors.

Pier-and-beam homes

Older homes in Waco neighborhoods like Sanger Heights, Brook Oaks and East Waco often sit on piers.

• Foundation vents with torn or missing screen
• Breaks and gaps in skirting, especially at corners and additions
• Holes where pipes, drains and ducts go up through the subfloor
• Gaps where a porch or addition meets the original house

The roofline, for roof rats

• Soffit returns at the corners
• Gable vents with damaged screen
• The joint where a lower roof meets a two-story wall
• Gaps between the fascia and the roof deck

Inside

The CDC’s list includes cabinet interiors, closet floors, the space behind appliances, pipe openings under sinks and near washers, water heaters and furnaces, dryer and floor vents and where the floor meets the wall.

What to seal with

Rodents gnaw, so the material matters.

Small holes

The CDC says to fill small holes with steel wool and put caulk around it to keep it in place, or use spray foam over it. The wildlife damage group says copper gauze or steel wool packed into a hole works as a temporary plug, and recommends mixing the mesh with quick-drying patching plaster or a similar compound and smoothing the outside for a longer-lasting fix.

Steel wool vs copper mesh

Both work. The wildlife damage group notes steel wool can leave rust stains, which matters on brick, stucco and painted siding, and in a humid crawl space. Copper mesh doesn’t rust and is easier to pack into irregular gaps, though it costs more. For a gap around a pipe outside, copper or stainless mesh plus sealant is a common choice.

Larger holes

The CDC lists lath screen or metal, cement, hardware cloth and metal sheeting for larger holes. The wildlife damage group gives specifics:

• Hardware cloth: half-inch mesh to exclude rats, quarter-inch mesh to exclude mice
• Galvanized sheet metal: 24 gauge or heavier for wall or pipe barriers, 22 gauge or heavier for kick plates and door edging
• Concrete: at least 2 inches thick if reinforced, about 3.75 inches if not

Doors

The wildlife damage group recommends no more than a quarter inch between the bottom of a door and the threshold. A door sweep or a new threshold fixes most gaps.

Weep holes

Don’t fill weep holes solid. They drain moisture from brick walls. Use vented inserts made for weep holes.

Garage doors

Garage doors are one of the most common gaps on Waco homes. The bottom seal wears and stops meeting the floor, and the corners where the door meets the frame often leave a gap a mouse can use. Replacement bottom seals and corner seals are inexpensive and close the gap.

What not to use alone

Foam sealant, caulk alone, plastic screen, wood and window screen all get chewed through. Use them only over or alongside metal.

Why it matters

Trapping clears the rodents inside. Sealing keeps the next ones out. A house with open quarter-inch gaps will keep getting mice no matter how many are trapped. The sealing checklist walks through the house room by room, and rodent exclusion covers what a technician seals. If you’ve sealed what you can find and still see signs, a mouse control visit can find what’s left.