Rat and mouse article

How Small a Hole Can Mice and Rats Fit Through?

Updated November 4, 2026

UF/IFAS says mice can get through a 1/4-inch gap and rats through 1/2 inch. Here's where those gaps hide on a Miami house and which materials hold up in this climate.

Smaller than most people think. UF/IFAS says mice can get through openings 1/4 inch wide and rats through cracks 1/2 inch wide. If a pencil fits in the gap, a mouse probably can too.

That's why rodent-proofing is about small openings, and why the material you seal them with matters as much as finding them. Here's where those gaps show up on Miami houses, and how steel wool, copper mesh, hardware cloth and foam compare.

Why they fit

A mouse's skull is the widest hard part of its body. If the head fits, the rest follows. Rats are bigger but just as flexible. They can also widen a gap by gnawing, so a 3/8-inch crack in soft material won't stay that size for long.

Climbing and jumping

Gap size isn't the only thing. UF/IFAS ENY-224 says rats can climb the inside of vertical pipes 1 1/2 to 4 inches across, the outside of pipes up to 3 inches across, and any pipe within 3 inches of a wall. They can jump 36 inches straight up and 48 inches across. On a Miami house, that makes downspouts, AC line sets and conduit along the wall into ladders to the roofline.

Where the gaps are on Miami houses

Look for openings at these spots:

• The open ends of barrel tile at the eaves
• Soffit panels that have pulled away, and vents with torn screens
• The hole where the AC line set enters the wall
• Gaps around hose bibs, gas lines, cable and phone lines
• Dryer vents with flaps that stick open
• Garage door bottom seals and corners
• Pipe holes under kitchen and bathroom sinks
• Seams where additions meet the original house

Steel wool

Steel wool is cheap and rodents don't like chewing it. The problem in Miami is rust. In this humidity, plain steel wool corrodes and falls apart, and it can leave rust stains on stucco. It works as a quick, temporary fill inside the house, but for anything long term, use something else.

Copper and stainless mesh

Copper mesh and stainless steel mesh do the same job as steel wool without rusting the same way. Pack it tightly into the gap, then cover it with sealant or mortar so it stays put and looks finished. This is what many providers use around pipes and the AC line.

Hardware cloth and sheet metal

For vents and larger openings, UF/IFAS ENY-224 recommends hardware cloth of 19 gauge or heavier with openings no larger than 1/4 inch, perforated metal of 24 gauge or heavier with the same opening size, and sheet metal of 26 gauge or heavier. Brick with mortared joints, cement mortar and concrete also hold up. Near the ocean or the bay, salt air corrodes metal faster, so galvanized or stainless materials last longer.

Foam and caulk

Expanding foam is easy and fills odd shapes, which is why it's stuffed around so many AC lines. Rats chew through it quickly. Caulk on its own doesn't stop them either. Use foam or caulk only as a finish over metal mesh or hardware cloth, never by itself.

Seal after trapping

Don't seal gaps while rodents are still inside, or you'll trap them in the walls or attic. Trap first, wait until catches and fresh droppings stop, then seal. Roof and soffit work is ladder work. If that isn't you, rodent exclusion covers what a provider does, and the rodent-proofing checklist covers the rest.

A 20-minute check of your own house

You can find most low gaps yourself:

1. Look under each exterior door for daylight
2. Check the garage door bottom seal and corners
3. Look under the kitchen and bathroom sinks where pipes enter the wall
4. Check where the AC line set and dryer vent leave the house
5. Walk around the outside and look at every pipe, wire and hose bib
6. From the ground, look at soffits, vents and tile ends with binoculars

Doors and garage seals

Doors are the most common low gap. A worn sweep or a bent garage door seal can leave far more than 1/4 inch, especially at the corners. Replace sweeps with ones that touch the threshold along the full width, and replace garage door bottom seals when they crack or flatten in the sun.

Low gaps around doors and pipes are reasonable DIY work. Roof, soffit and tile work is ladder work, and a provider will catch openings that are hard to see from the ground.

Related services: Rodent Exclusion, Mouse Control, Garage Rodent Control