What size hole can a mouse fit through?
About 1/4 inch. If a pencil fits, a mouse fits.
How do I find where mice are getting in?
Reviewed by William "Willy" Vizcarrondo, NYS DEC Certified Commercial Pesticide Applicator C1825832. Updated September 23, 2026.
Short answer
Walk the outside of the house at foundation level and at the roofline looking for any gap 1/4 inch or larger, the width of a pencil, then check the same spots inside. The usual entries are where pipes, cables, and gas lines enter, the gap under the garage door, cracks at the foundation sill, dryer and crawl space vents, and gaps at eaves and soffits (CDC).
Outside
The CDC's list: roof rafters, gables, eaves, and soffits; windows and doors without weather stripping; gaps between the foundation and the ground; attic and crawl space vents; and the openings for electrical, plumbing, cable, and gas lines. Add the garage door seal, which is the single most common entry on Long Island splits and ranches, and any spot where a tree limb or a wire touches the house, because deer mice and white-footed mice enter high up.
Inside
Under, behind, and inside kitchen cabinets; closet floors; around the fireplace; behind appliances; the pipes under sinks and behind the washing machine; water heater and furnace pipes; floor vents; electrical and utility line entries; attics; basements; floor drains; and where the floor meets the wall (CDC). Droppings, grease marks, and chewed material mark the active routes; follow them to the wall and look for the hole.
Sealing it right
Small holes get steel wool packed in and held with caulk or foam over it; large holes get lath screen, metal, cement, hardware cloth, or metal sheeting (CDC). Mice gnaw through plastic screening, rubber, vinyl, foam alone, and wood (UC IPM), so foam by itself is a temporary patch, not a fix.
On Long Island
The three entries we find most in Nassau and Suffolk are the gap where the garage door meets the concrete, the hole around the AC line set or gas line into the basement, and the sill plate on fieldstone foundations in older North Shore homes. Every Bobby Bugs mouse job includes finding and sealing them, which is why it carries a 60-day guarantee.
Related
Related questions
Does spray foam keep mice out?
Not by itself. Mice chew through foam. Pack steel wool into the hole first and foam or caulk over it, or use metal for bigger openings.
Can mice climb walls?
Yes. They climb brick, siding, wires, and tree limbs, which is why roofline gaps and attic vents matter as much as the foundation.
Sources
Still not sure? Call (516) 509-6001 and we'll tell you what you're dealing with.