Mice or rats? How to tell

The usual suspects

House Mouse Mus musculus The house mouse is a small, adaptable rodent that lives close to people in homes, schools and food businesses, eating and contaminating food, gnawing wiring and carrying disease. Norway Rat Rattus norvegicus The Norway rat, also called the brown or sewer rat, is a large, burrowing rat found throughout the contiguous United States wherever people live. Roof Rat Rattus rattus The roof rat, also called the black or ship rat, is a sleek, agile climber that nests above ground in trees, ivy, attics and wall voids. Deer Mouse Peromyscus species (shown: P. maniculatus) The deer mouse is a wild mouse with a white belly and a furred, two-toned tail that sometimes moves into cabins, sheds and homes. In the United States it spreads the most common hantavirus that causes hantavirus pulmonary syndrome (HPS), a severe and potentially deadly lung disease, so the droppings and nests it leaves need careful cleanup.

Droppings: size and shape

CDC describes rodent droppings as pointed at one end and often containing hair. Because old droppings don't tell you whether rodents are still there, clean them up the safe way and check again: droppings that show up after cleaning mean the rodents are active.^[12]

• House mouse: droppings average about 1/4 inch long, found along runways, in feeding areas and near shelter.^[5]
• Roof rat: about 1/2 inch long with pointed ends.^[9]
• Norway rat: as large as 3/4 inch long and 1/4 inch across, with blunt ends. A rat leaves about 40 to 50 droppings a day, usually clumped in small groups, while other rodents tend to scatter theirs. Fresh droppings are soft.^[7]
• Not a rodent: cockroach droppings are usually 1/8 to 1/4 inch long, and under a magnifying glass show lengthwise ridges and squared-off ends. Bat droppings contain bits of insects and crumble easily.^[5]

Size, tail, ears and eyes

Sources give slightly different weights. UC IPM puts an adult house mouse at about half an ounce, roof rats at 5 to 10 ounces and Norway rats at 7 to 18 ounces. ICWDM gives 0.4 to 0.9 ounce for house mice, about 5 to 9 ounces (up to 12) for roof rats, and an average of 12 to 16 ounces for adult Norway rats.^{[2][1][5][9][7]}

• House mouse: small and slender with a slightly pointed nose, small black, somewhat bulging eyes, large, sparsely haired ears, and a nearly hairless tail with visible scale rings. The body is 3 to 4 inches long and the tail 2 to 4 inches. Fur is gray-brown with a gray or buff belly.^[5]
• Deer mice, white-footed mice and jumping mice look similar, but their white bellies contrast sharply with the brown fur on the back.^[5]
• Roof rat: body 6 to 8 inches with a 6- to 8-inch scaly tail, about 16 inches in all, and soft, smooth brown fur mixed with black.^[9]
• Norway rat: coarse fur, usually brown or reddish gray above and whitish gray below, and a tail that doesn't reach the tip of the nose.^[7]

Roof rat or Norway rat?

• Roof rats are sleek and agile with a pointed muzzle, ears that reach the eyes when folded over, and a uniformly dark tail that reaches at least to the snout when pulled over the back. They weigh 5 to 10 ounces.^[1]
• Norway rats are large and robust with a blunt muzzle, ears that don't reach the eyes, and a tail that reaches only the middle of the skull, dark above and pale below. They weigh 7 to 18 ounces.^[1]
• Roof rats nest up high: attics, wall and ceiling voids, cabinets, trees and ivy. Norway rats burrow along foundations and under woodpiles and trash, and indoors stay in basements and ground floors.^[1]
• Woodrats (pack rats) have furry tails; Norway and roof rats have scaly, mostly hairless tails.^[1]

Gnaw marks and entry holes

Both groups gnaw, and rat tooth marks are wider, but the published widths differ. UC IPM gives about 1/6 inch (4 mm) for rats and 1/12 inch (2 mm) for a mouse's scratchy marks. ICWDM gives 1/16 inch for rats and 1/32 inch for mice, with rat grooves nearly twice the size of mouse marks.^[1][5][7]

• Holes made by mice are often 1 1/2 inches across or smaller, and hole size helps separate mouse gnawing from rat gnawing.^[5]
• Active Norway rat burrows and entrance holes are 2 to 3 inches wide, clean of debris, with rat hairs around the opening. Look next to walls and buildings, along fences, and under bushes and debris.^[7]
• Fresh piles of wood shavings, insulation or other chewed material mean the gnawing is recent.^[5][7]
• Roof rats living in an attic and feeding outdoors may limit their damage indoors to insulation torn up for nests or gnawed electrical wiring.^[11]

Signs to look for

• Droppings near pet food, food storage and recycling bins; fresh gnaw marks; and tracks.^[1][2]
• Greasy smudge marks where rats rub against beams, pipes and walls, and burrows under compost piles, garbage cans and garden plants.^[1]
• House mouse nests of finely shredded paper or fiber in sheltered spots, and a musky odor with large or long-term infestations.^[2]
• Noises at night: both are mostly nocturnal, and roof rats are often seen running along utility lines and fence tops.^[1][2]
• Tracks and tail marks show up in dust or mud. To check a pathway, ICWDM suggests a patch of flour rolled smooth and left overnight.^[5][7]
• Smell can help: a musky odor points to mice rather than rats.^[5]
• Rub marks from mice may be less obvious than those rats leave.^[5]
• In fruit trees, roof rats often eat the pulp out of oranges still on the tree and leave the empty rind; with lemons they may eat only the rind.^[11]

How they behave

Mice are nibblers. UC IPM says they eat up to 20% of their body weight a day in about 200 small meals a night, and seldom go more than 30 feet from their nest and food. They climb any rough vertical surface, run along wires and ropes, and can jump 12 inches from the floor onto a flat surface. Where mice live outdoors, they often move into homes in autumn as nights get colder.^[2]

Rats range farther and are warier. UC IPM says Norway rats may forage anywhere from 100 to 500 feet from their nests, depending on local conditions; ICWDM says a Norway rat covers an area about 100 to 150 feet across each day and seldom goes more than 400 feet from its burrow, and that roof rats will travel 100 to 300 feet for food when they need to.^[1][8][10]

Rats quickly notice and avoid new objects and new foods. ICWDM says traps and bait stations are often avoided for several days, and UC IPM notes this fear seems stronger in roof rats than in Norway rats. Mice are less afraid of new objects than rats.^{[1][8][10][6]}

Norway rats get into buildings by gnawing, climbing, jumping and swimming, including through sewers and in through toilets or broken drains. Although most are good climbers, Norway rats tend to live on the lower floors of buildings.^[1][8]

Why it changes what you do

Once you know which one you have, follow the matching plan: the mouse guide for sealing to 1/4 inch and close-set traps, the rat guide for rat-proofing and for setting traps that wary rats will accept.^[2][1][8]

• Sealing: close every gap larger than 1/4 inch to keep mice out, using steel wool as a temporary plug and metal or concrete for permanent repairs. Don't use lightweight chicken wire for rats; they pass through it.^[2][1]
• Trap size and placement: house mice seldom go more than 30 feet from the nest, so set mouse traps close together where you see signs. For Norway rats set traps along walls and in dark corners; for roof rats, also up high.^[2][1]
• Snap traps are the safest, most effective and most economical way to trap rats; nuts, dried fruit, bacon or dry pet food make good bait.^[1]
• Disease: house mice can carry lymphocytic choriomeningitis virus, spread through fresh urine, droppings, saliva or nesting material. Deer mice carry hantavirus; never sweep or vacuum droppings dry.^[2][3][4]

Common questions

Could a small rodent be a young rat rather than a mouse?

Yes. A young rat can be confused with a mouse, but young rats have large heads and feet for their bodies, while an adult mouse's are proportionately much smaller.^[1]

Why does a rat ignore a trap for days?

Rats avoid new objects, so traps and bait stations are often left alone for several days. ICWDM notes that rats may check out baited, unset traps after a short wait, which helps them get over that fear.^[8]

Is it a mouse or a deer mouse?

Deer mice have white bellies that contrast sharply with brown fur on the back; house mice are gray-brown with a gray or buff belly. It matters because the deer mouse spreads the most common hantavirus that causes HPS in the United States.^[5][3]

Sources

1. University of California Statewide IPM Program, “Rats (Pest Notes, Home and Landscape).” ipm.ucanr.edu/home-and-landscape/rats/ (accessed 2026-11-10)
2. University of California Statewide IPM Program, “House Mouse (Pest Notes, Home and Landscape).” ipm.ucanr.edu/home-and-landscape/house-mouse/ (accessed 2026-11-10)
3. Centers for Disease Control and Prevention, “About Hantavirus.” www.cdc.gov/hantavirus/about/index.html (accessed 2026-11-10)
4. Centers for Disease Control and Prevention, “How to Clean Up After Rodents.” www.cdc.gov/healthy-pets/rodent-control/clean-up.html (accessed 2026-11-10)
5. Internet Center for Wildlife Damage Management, “House Mice: Identification.” icwdm.org/species/rodents/house-mice/ (accessed 2026-11-10)
6. Internet Center for Wildlife Damage Management, “House Mouse Damage Prevention and Control Methods.” icwdm.org/species/rodents/house-mice/house-mouse-damage-prevention-and-control-methods/ (accessed 2026-11-10)
7. Internet Center for Wildlife Damage Management, “Norway Rats: Identification.” icwdm.org/species/rodents/norway-rats/ (accessed 2026-11-10)
8. Internet Center for Wildlife Damage Management, “Norway Rat Biology.” icwdm.org/species/rodents/norway-rats/norway-rat-biology/ (accessed 2026-11-10)
9. Internet Center for Wildlife Damage Management, “Roof Rats: Identification.” icwdm.org/species/rodents/roof-rats/ (accessed 2026-11-10)
10. Internet Center for Wildlife Damage Management, “Roof Rat Biology.” icwdm.org/species/rodents/roof-rats/roof-rat-biology/ (accessed 2026-11-10)
11. Internet Center for Wildlife Damage Management, “Roof Rat Damage Identification.” icwdm.org/species/rodents/roof-rats/roof-rat-damage-identification/ (accessed 2026-11-10)
12. Centers for Disease Control and Prevention, “Rodent Control.” www.cdc.gov/healthy-pets/rodent-control/index.html (accessed 2026-11-10)