Quick answer
Droppings settle it in most houses. Mouse droppings are the size of a grain of rice, 1/8 to 1/4 inch, with pointed ends, scattered by the dozen along cabinet backs and under the sink. Norway rat droppings are about 3/4 inch with blunt ends, fewer of them, grouped along a wall in the basement or garage. If you also see a hole 2 to 4 inches across in the dirt by the foundation, a deck, or a shed, it is rats. Mice live inside the walls within about 30 feet of the food; Norway rats live in burrows outside and come in to eat. The two take different traps, different materials, and different plans, and getting the animal wrong is the most common reason a rodent problem on Long Island lasts all winter.
Look at the droppings, then the size of the hole. Rice-grain droppings with pointed ends are mice. Capsule shaped droppings close to 3/4 inch with rounded ends are Norway rats. A gap you could push a pencil through, 1/4 inch, lets a mouse in; a rat needs 1/2 inch, about the size of a quarter. A 2 to 4 inch burrow opening in soil along the foundation is a rat burrow, and no mouse made it.
After thirty five plus years of crawling through Nassau and Suffolk basements, the honest version is that most homeowners who call about "rats" have mice, and a smaller number who call about "a big mouse" have a rat. UC IPM, the University of California's pest program, puts house mouse infestations at roughly 10 to 20 times more common than rat infestations, mainly because mice fit through gaps rats cannot. The sections below walk through the seven tells in the order I check them on a job, and then why the answer changes everything that happens next.
Size and shape. The University of Kentucky's rodent control guide gives the numbers: adult Norway rat droppings are about 3/4 inch long and capsule shaped with blunt ends, roof rat droppings are about 1/2 inch and pointed on one end, and house mouse droppings are 1/8 to 1/4 inch with at least one pointed end. Mouse droppings look like black rice. Rat droppings look like a fat capsule or a raisin.
Count and placement are the second tell. Mice forage constantly in a small area, so you find dozens of droppings along a cabinet back, in a drawer track, under the sink, or on top of a basement sill plate. Rats leave fewer, larger droppings in small groups along their travel line against a wall, near pet food, or in the garage. Fresh droppings of either animal are dark and soft; old ones are gray, dry, and crumble. The CDC's advice for telling an active problem from an old one is to clean the area safely and see whether new droppings appear. If they do, you have traffic right now. We cover the safe cleanup method in how do I clean up mouse droppings safely, and the short rule is never dry sweep or vacuum them.
Yes, and the easiest way to see it is proportion, not length. UC IPM describes the adult house mouse as 5 to 7 inches long including a 3 to 4 inch tail, weighing about half an ounce, with relatively large ears and small black eyes. An adult Norway rat weighs 7 to 18 ounces, has a blunt muzzle, small ears that do not reach the eyes when folded forward, and a thick scaly tail that is shorter than its head and body combined. A rat is not a big mouse. A rat is 15 to 30 times heavier.
The one that fools people is the young rat. A juvenile Norway rat can be mouse sized, and that is the "big mouse" call I get every fall. UC IPM's tell is the head and feet: a young rat has a large head and large feet for its body, while an adult mouse's head and feet are small and in proportion. A mouse looks finished. A young rat looks like it is wearing its parents' shoes. If you have a photo of the animal, text it to (516) 509-6001 and we will tell you which it is.
Rats chew bigger, rub greasier, and need a bigger hole. UC IPM measures the tooth marks: rats leave gnaw marks about 1/6 inch wide, while a mouse leaves finer "scratchy" marks about 1/12 inch. A chewed corner on a cereal box with fine scratches is a mouse. A gnawed hole through the bottom of a plastic garbage can, a door sweep, or a wooden garage door with clean, wide tooth grooves is a rat.
Rub marks are the next clue. Both animals travel the same path every night and their oily fur leaves a dark smudge where the body touches a surface. On a mouse job the smudges are faint and low, along a baseboard or the top of a foundation wall. On a rat job UC IPM describes smudge marks on beams, rafters, pipes, and walls, and they are wide and obvious because the animal is bigger and heavier. Rats also have the strength to shift things: a chewed through dog food bag, pet food bowls dragged, or a dead rat your dog brought in all point one way.
Then the hole. The University of Kentucky's guide says rats can enter through a crack the size of a quarter, about 1/2 inch, and UC IPM says young rats squeeze under a door with only a 1/2 inch gap. Mice get through anything over 1/4 inch. That is why a rodent inspection on a Long Island house is a measuring job: every gap over a quarter inch goes on the list, because sealing to a mouse standard keeps both animals out. Spray foam alone keeps neither out, which we covered in what can mice and rats chew through.
Mice set up inside the house and rarely leave it. UC IPM notes house mice seldom venture far from their shelter and food supply, and the trap spacing rule, no more than about 10 feet apart, follows from that. A mouse nest is a ball of finely shredded paper or insulation in a wall void, a kitchen kick plate, the back of a basement workbench, a garage shelf, or the insulation above a drop ceiling. The musky odor in a closed cabinet is a mouse tell on its own.
Norway rats do the opposite. UC IPM describes a burrowing rodent whose burrows run along building foundations, beneath rubbish or woodpiles, and around gardens, with a nest inside lined with leaves, shredded paper, or cloth, an entrance, and an escape hole. The nightly foraging range is 100 to 500 feet, so the rats eating out of your garbage may live under the neighbor's shed. When Norway rats do come inside, UC IPM says they usually stay in the basement and ground floor. A rat in the basement and a burrow outside is the normal Long Island picture, and the yard is where the job is won. We covered the usual attractants in what attracts rats to a yard.
One more location tell: noise. Light, constant scratching at night in a kitchen wall or ceiling is mice. Heavier movement at night in a basement, a crawl space, or under a deck, with droppings to match, is more likely a rat. Daytime noise in the attic is usually squirrels, not rodents, and that is a wildlife job rather than an extermination job.
The Norway rat, also called the brown rat or sewer rat. The New York City Health Department teaches in its Rat Academy that the only type of rat found in the five boroughs is the Norway rat, and the rat we pull from Nassau and Suffolk basements is the same animal. The roof rat, the smaller climber with a tail longer than its body that UC IPM describes living in trees and attics, is a warm climate rat. In thirty five plus years on Long Island I have never had a confirmed roof rat on a job, and if you read a national pest site telling you to set traps in the attic for roof rats, that advice was written for Florida or California.
That matters because it simplifies the search. The Long Island rat burrows. It lives low. It travels along foundations, fence lines, and the base of hedges, and it concentrates where there is water, restaurant trash, bird seed, chicken coops, and compost: the waterfront villages, Route 110 and the restaurant strips, older homes with fieldstone foundations, and any block where one neighbor feeds the birds every day. If your rodent evidence is in the attic, think mice or squirrels first and rats last.
Because the trap, the bait, the placement, the materials, and the timeline are all different, and using the mouse version on a rat is how homeowners spend four months at it. UC IPM says it plainly: mice and small juvenile rats often will not trigger a rat snap trap, so mouse sized traps set 10 feet apart on the runways are the mouse job. Rat traps are larger, set 10 to 20 feet apart, placed at right angles to the wall with the trigger end touching it, and UC IPM recommends a dozen or more for a heavily infested home.
Rats are also wary of new things. UC IPM says rats quickly detect and avoid new objects and new foods, and often avoid traps and baits for the first several days or even weeks after placement. Mice are curious; rats are suspicious. That is why a rat job includes pre baiting unset traps until the bait is taken, and why exterior tamper resistant bait stations at the burrows, serviced by a licensed applicator, are usually part of it. A mouse job is traps indoors plus sealing, and we do not use poison inside the house for either animal, because both UC IPM and Penn State warn that rodents poisoned indoors die inside walls and smell for weeks.
The sealing standard is the same, 1/4 inch, but the scope is not. A mouse job is mostly the house: the gap around the gas line, the dryer vent with no screen, the garage door bottom. A rat job adds the yard: the burrow at the foundation, the gap under the shed, the deck skirting, the compost pile, the dog food on the back step. Both jobs at Bobby Bugs start with a free written quote and carry a 60 day written guarantee, and the quote names the animal so you know what you are paying for. That is the difference between the mice exterminator and rat exterminator pages on this site, and it is why we ask you to send a photo of the droppings before anyone shows up.
Calling a rat a mouse, and buying the small traps. The traps never fire, the homeowner concludes the traps are bad, and the burrow in the yard keeps producing rats. If a rat trap is what the droppings call for, use rat traps, use plenty of them, and leave them baited but unset for a few days first.
Putting poison in the walls. Whether it is mice or rats, a rodent that eats bait indoors dies indoors. UC IPM and Penn State both say so, and the smell that follows is the call I get three weeks later. Bait belongs in locked stations outside, placed by a licensed applicator who follows the label, and nowhere else.
Sealing with foam, caulk, or steel wool alone. UC IPM says plastic screening, rubber, vinyl, insulating foam, and wood are unsuitable for plugging holes used by mice, and rats chew through plastic, foam, wood, and caulk. Metal and cement hold. Stainless steel scouring pads are a temporary plug at best.
Treating the yard as somebody else's problem. A rat job that stops at the foundation wall is a rat job that comes back, because the burrow is 20 feet away under the woodpile. Garbage in metal cans with secured lids, pet food brought in at night, bird feeders moved away from the house, and a 2 foot gap between shrubs and the foundation are the rat prevention list straight from UC IPM, and they are what keep the 60 days quiet.
Ignoring one mouse. UC IPM says a female can have 5 to 10 litters a year of about 5 or 6 young, born 19 to 21 days after conception, and the young are breeding in 6 to 10 weeks. The one mouse in October is a family by Thanksgiving. We covered the math in I saw one mouse, how many do I have.
Mouse droppings are 1/8 to 1/4 inch, the size of a grain of rice, with pointed ends, found by the dozen. Norway rat droppings are about 3/4 inch, capsule shaped with blunt ends, found in smaller groups along a wall. Those numbers come from the University of Kentucky's rodent control guide.
It can be, and that is the animal that fools people. UC IPM's tell is proportion: a young rat has a large head and large feet for its body, while an adult mouse's head and feet are small and in proportion. If the "mouse" looks clumsy and big footed, it is probably a young rat, and there are more where it came from.
A mouse fits through a gap slightly larger than 1/4 inch, the width of a pencil. A rat needs about 1/2 inch, the size of a quarter. Sealing every gap over 1/4 inch with metal or cement keeps both out.
Usually. Mice are light, fast, constant scratching at night, often in kitchen walls and ceilings. Rats are heavier and slower, more often low in the house, in a basement, crawl space, or under a deck. Daytime noise in the attic is more likely squirrels than either rodent.
The Norway rat. The NYC Health Department teaches that it is the only rat found in the five boroughs, and it is the rat on Long Island too. It burrows outside along foundations and comes in low, through basements and garages, not through the roof.
No. UC IPM states that commensal rats and mice do not carry or transmit rabies. They do carry other diseases, including salmonella, leptospirosis, and rat bite fever, so droppings should be cleaned with gloves and disinfectant, never dry swept.
It is uncommon. UC IPM finds mouse infestations 10 to 20 times more common than rat infestations, and in my experience a house with both usually has rats living outside in burrows and mice living inside the walls. A single inspection sorts it out, and the sealing list keeps both out.
Related reading
Sources: UC IPM, House Mouse; UC IPM, Rats; University of Kentucky Entomology, Rodent Control; CDC, Controlling Wild Rodent Infestations; CDC, Seal Up; Penn State Extension, Is There a Mouse in the House?; NYC Department of Health and Mental Hygiene, Rat Academy training.
Still not sure? Call (516) 509-6001 and we'll tell you what you're dealing with.
This article is general pest information for Long Island homeowners, not a diagnosis of your specific situation. Pest behavior varies by home, season, and species, and every job is quoted in writing before any work begins. Pesticide applications are performed by licensed New York State applicators in accordance with label requirements.