Rat vs Mouse: Size, Droppings and Signs

A rat is much bigger than a mouse: an adult Norway rat weighs roughly ten times as much as a house mouse and has a thick body, blunt nose, small ears and a heavy tail. A mouse is small and slender, with a pointed nose, big ears and a thin tail as long as its body. You will often not see either, though, so the droppings are the more useful evidence. They differ clearly in size and shape, and so do the holes, the gnawing and the way each one behaves around traps.

The animals side by side

Feature	House mouse	Norway rat	Roof rat
Body length (without tail)	About 2 1/2 to 3 1/2 inches	About 7 to 10 inches	About 6 to 8 inches
Weight	About 1/2 to 1 ounce	About 7 to 18 ounces	About 5 to 10 ounces
Nose	Pointed	Blunt	Pointed
Ears	Large for the head	Small, close to the head	Large, thin
Tail	Thin, about as long as the body	Thick, shorter than head and body	Longer than head and body
Build	Slender, small feet	Heavy, stocky	Sleek, slender
Where in a house	Anywhere: walls, cabinets, drawers, appliances	Basements, crawl spaces, ground floors, burrows outside	Attics, upper walls, trees, climbing

Norway rat or roof rat

The US has two common rats, and they live in different parts of a building. Norway rats, also called brown rats or sewer rats, are heavy burrowers. They live in the ground along foundations, under slabs and in sewers, and come into basements, crawl spaces and ground floors. Roof rats, also called black rats, are lighter and agile climbers. They live in trees, ivy, attics and upper walls, travel along fences, power lines and branches, and are most common in coastal and southern states, including California, the Gulf Coast and Florida. A rat in the attic in Phoenix or Miami is likely a roof rat; a rat burrowing under the shed in Chicago is a Norway rat. The fix follows from it: trapping and sealing low for Norway rats, trimming branches back from the roof and sealing the roofline for roof rats.

Young rat or adult mouse

The most common mix-up is a juvenile rat taken for a large mouse. Proportions solve it. Young rats have heads and feet that look too big for their bodies, like a puppy's paws, and their tails are already thick. Adult mice have small heads, delicate feet, oversized ears and long thin tails. If it looks like a baby, it is probably a young rat; adult mice look like miniature adults.

The droppings

Droppings	Size	Shape	Points to
Many tiny dark pellets scattered along walls and in drawers	About 1/8 to 1/4 inch	Pointed at one or both ends, like rice grains	Mice
Fewer, larger pellets in groups	About 3/4 inch	Blunt ends, capsule-shaped	Norway rats
Medium pellets in attics and along high runways	About 1/2 inch	Pointed ends, slightly curved	Roof rats
Tiny specks like ground pepper in cabinets	Under 1/8 inch	Ridged or blunt	Small cockroaches, not rodents
Shiny pellets that crumble into insect parts	Similar to mouse	Pointed	Bats; crumble when crushed

Fresh droppings are dark and soft; older ones turn gray and crumbly. Mice leave far more droppings, often 50 or more a day each, scattered wherever they travel. Rats leave fewer, larger ones along their regular routes.

Other signs

• Entry holes. Mice need a gap of only about 1/4 inch, roughly a pencil's width. Rats need about 1/2 inch, roughly a quarter. Holes with greasy, dark rub marks around them point to rats, which follow the same paths and leave body oil on walls and pipes.
• Gnawing. Mouse tooth marks are tiny and fine. Rat gnawing leaves larger, rougher marks and can chew through soft metal, wiring and plastic pipe.
• Sounds. Light scratching and skittering in walls is typical of mice. Heavier thumping and gnawing, especially in the attic at night, suggests roof rats.
• Burrows outside. Rats dig burrows about 2 to 4 inches across along foundations, under slabs and in garden beds, often with a fan of soil. Mice rarely burrow outdoors near a house.
• Nests. Mice build small balls of shredded paper, insulation and fabric inside walls, drawers and appliances.

Why waiting is expensive

Both breed quickly. A female house mouse can have a litter of five or six young about three weeks after mating and can become pregnant again almost immediately, producing several litters a year. Rats are similar, with litters of six to twelve. Young mice can breed at six weeks old. A pair of mice in October can become dozens by spring, which is why a single sighting is worth acting on right away rather than waiting to see if more appear.

Why the difference changes the fix

Mice are curious. They investigate new objects, travel only 10 to 30 feet from the nest, and take traps readily when they are set every few feet along walls. Rats are cautious and avoid new things for days, a behavior called neophobia. They travel much farther, often 100 feet or more, and a trap set in the wrong place or used too early gets ignored. With rats, set traps unarmed with bait for a few days so they get used to them, then arm them. Place all traps perpendicular to walls, with the trigger toward the wall, where rodents run.

Bait matters less than placement. Peanut butter, chocolate and dried fruit work for both. Larger snap traps are needed for rats; a mouse trap will only injure a rat.

What does not work

Plug-in ultrasonic repellers are widely sold and poorly supported: rodents get used to the sound within days, and it does not pass through walls or furniture. Peppermint oil, dryer sheets and mothballs do little against an established population, and mothballs are toxic to people and pets in enclosed spaces. Poison baits kill, but rodents often die inside walls where they smell for weeks, and pets or wildlife can be poisoned by eating the bait or a poisoned rodent. Sealing entry points and trapping are slower and more reliable.

Health and cleanup

Both carry diseases. House mice and rats can spread salmonella through droppings on food surfaces, rats carry leptospirosis, and deer mice, the white-bellied mice found in rural sheds and cabins, can carry hantavirus. When cleaning droppings or nests, open windows for at least 30 minutes, wear gloves, spray droppings with a disinfectant or a bleach solution of about one part bleach to ten parts water, let it soak for five minutes, then wipe it up with paper towels and bag it. Do not sweep or vacuum dry droppings.

Photo the droppings

A photo of droppings with a coin or ruler next to them is often more useful than a blurry photo of the animal. The animal scat identifier separates mouse, rat, cockroach, bat and squirrel droppings from a clear, well-lit close-up, because size and shape are what differ. Without something for scale, mouse and roof rat droppings are hard to tell apart; always include a reference. For the animal itself, a side view that shows the ears, nose and tail settles it.

Call a professional when droppings keep appearing after a few weeks of trapping and sealing, when you hear activity in several parts of the house, when rats are burrowing along the foundation, or in apartment buildings where rodents travel between units. Exclusion work, sealing a whole building, is the part most people find hardest to do themselves.

For the outdoor rodent people mistake for a mouse, the vole vs mouse guide covers the ones in your lawn.