Rats are diverse rodents whose body mass varies dramatically across species, environments, and maturity levels. The weight of a rat is not a single fixed number but rather a wide spectrum influenced by whether the animal lives in the wild or in a controlled domestic setting. The differences in habitat and selective breeding account for the significant range in body mass observed.
Average Weight of Wild Rat Species
The two most widely recognized wild rat species are the Norway rat (Rattus norvegicus) and the roof rat (Rattus rattus). The Norway rat, also known as the brown rat or sewer rat, is the larger and heavier of the two, possessing a stockier build. An adult Norway rat typically weighs between 7 and 18 ounces (200 to 510 grams), though most wild specimens weigh around 12 to 17 ounces (340 to 480 grams). These ground-dwelling rodents have been reported to reach maximum weights near 2 pounds (approximately 900 grams) in exceptional cases.
In contrast, the roof rat, often called the black rat or ship rat, is more slender and agile. Adult roof rats generally weigh much less, with a typical range of 5 to 9 ounces (140 to 255 grams). This lighter weight and streamlined body suit their preference for living in elevated spaces like attics and trees.
Weight Range for Domesticated Pet Rats
Domesticated rats, commonly referred to as Fancy Rats, are descended from the Norway rat but have been selectively bred to be significantly larger and more robust than their wild ancestors. Consistent nutrition, veterinarian care, and a lack of predation contribute to their increased body size. Adult pet rats exhibit clear sexual dimorphism, with males and females having distinct size differences.
Adult male pet rats, called bucks, are substantially heavier than females, typically weighing between 450 and 650 grams (1 to 1.4 pounds). Females, or does, usually maintain a lower body mass, ranging from 350 to 450 grams (0.8 to 1 pound) in adulthood. A large male pet rat can easily outweigh a wild male Norway rat by over 100 grams.
Factors Influencing Rat Body Mass
Sex is a primary determinant, as male rats naturally attain a greater body mass than females, a pattern that holds true for both wild and domesticated animals. Diet and access to food sources also play a major role in body mass. Wild rats living in urban environments often consume high-calorie waste, which can lead to larger sizes than those with less consistent food availability.
Pet rats can become overweight if fed an improper diet high in fat, such as seed mixes, rather than a balanced pelleted ration. Age also affects body mass, with rats continuing to gain weight until they reach full maturity. Full maturity can take up to seven or eight months in some strains, after which weight gain slows significantly.