Appearance

If you actually see the rodent, you should be able to determine what it is. House mice are not only smaller than Norway and Roof rats, but they look different as well. Mice are between three and four inches long, and they have thin tails that are two to four inches long. They are typically grey, have small bodies and have prominent ears. Their heads are small and pointed, and they have small feet. 

Rats are larger than mice. Norway rats are the larger of the two commonly found in houses, and they can reach as long as ten inches; their tails are thick and slightly shorter than their bodies. Whereas mice typically weigh only a few ounces, Norway rats usually weigh 18 ounces or more. They have thicker, stouter bodies than both mice and Roof rats. Norway rats have smaller ears and eyes than Roof rats, and they are usually brown or grey in color. 

Roof rats are typically around eight inches long, and their thick tails are a little longer than their bodies. They have a more slender body than a Norway rat, and their eyes and ears are more prominent. Roof rats may be black, as they are commonly called black rats, but they can be dark to light brown as well. 

Droppings

Although all rodent droppings are typically black, the droppings of rats and mice look different, so you can use the droppings to determine which rodent is infesting your home. Mice tend to leave more droppings than rats, and the droppings are smaller. Each dropping is tiny, about the size of a grain of rice. They are also hard and pointed. Rats leave larger droppings, measuring about one-half to three quarters of an inch long. They also appear crescent shaped, rather than straight like mouse droppings. Rats typically don’t leave as many droppings as mice. 

The presence of droppings may not mean you have a current infestation. Fresh droppings will be shiny and putty-like to the touch, but older droppings will be dull and may even crumble when you touch them. 

Footprints

Another way to determine which rodent is infesting your home is to look at the footprints they are leaving in dusty areas. If there is no dust, sprinkle some talcum powder or flour lightly in the area where you think they are traveling. Mice have smaller feet and smaller footprints, measuring about one-quarter of an inch. Both species of rats have larger feet, and their footprints are typically three-quarters to one full inch long. You are not likely to confuse baby rat prints with mice prints because a young rat won’t be venturing out on its own; larger footprints would show up as well.

Grease Marks

Check along the bottoms of your walls for marks. Both mice and rats carry oil and dirt on their skin; however, rats are more likely to follow the same path through your home. They stick close to the wall as well, so they will leave behind a greasy trail as they go along the wall. Mice may leave greasy rub marks if they exit or enter a hole, but it won’t be along the baseboards of your walls 

Urine Pillars

Mice will often urinate in one particular area. When their urine dries, it leaves behind a pillar of dirt and oil. These tiny pillars may have a small urine trail leading to it. If you find urine pillars, then you have a mouse infestation rather than rats.

Gnaw Marks

All rodents gnaw to keep the size of their incisors down. However, rats may gnaw on walls, but mice do not. While you may find evidence of shredded paper and other materials from any rodent, if you find gnaw marks on your walls, then you have a rat infestation. If a rat gnaws a hole in your wall, it will have rough, ragged edges.

Visibility

Rats tend to only venture out at night. Mice, on the other hand, are more curious and explorative. If you see a rodent during the daytime or in a well-lit area at night, it is likely a mouse. You will rarely encounter a rat during the day in your home.