A mouse needs an opening as small as about a quarter of an inch (around 6 mm) wide to enter a building. Mice have flexible, collapsible skeletons without rigid collarbones, which allow them to flatten their bodies and squeeze through very narrow gaps. Essentially, if a mouse can fit its head through a hole, its entire body can follow. More specific measurements from experiments and expert sources show that a mouse can fit through holes roughly 5/8 inch (about 16 mm) in diameter or even slightly smaller, although sub-1/2 inch holes (about 13 mm) are usually too small for an adult mouse. Gaps smaller than this can sometimes still be exploited if the mouse can flatten or elongate its body enough. So to prevent mice from entering, it is critical to seal any holes or gaps larger than approximately 1/4 inch (6 mm) since even very small openings can be accessed by mice, especially gaps under doors or around utility lines entering buildings. Using steel or copper wool combined with sealants can effectively block these entry points. In summary:
- Minimum opening size for mouse entry: approximately 1/4 inch (6 mm) wide
- Experimentally confirmed squeeze-through holes: around 5/8 inch (16 mm) or slightly less
- Smaller holes (~1/2 inch or 13 mm) typically prevent adult mice from entry
- Mice can flatten their bodies to fit through tighter spaces
Sealing any holes or gaps larger than 1/4 inch is necessary for rodent- proofing a building. This includes cracks in walls, gaps under doors, and spaces around pipes or utility lines.
