The size of a “rat” varies significantly because the term refers to numerous species of rodents in the genus Rattus and other related genera. For most people, the inquiry focuses on the two species that interact most closely with humans globally, often called commensal rats. Understanding the true dimensions of these common pests requires separating their typical size range from extreme outliers and cases of mistaken identity.
Average Size Range of Common Rat Species
The two most widespread pest species, the Norway Rat and the Roof Rat, display distinct physical differences in their average adult size.
The Norway Rat (Rattus norvegicus) is the larger and more robust species, often referred to as the brown or sewer rat. An adult typically weighs between 140 and 500 grams (5 to 17.6 ounces). Its body length, measured from nose to the base of the tail, generally falls between 22 and 26 centimeters (8.6 to 10.2 inches). Total length, including the tail, can reach up to 40 centimeters (16 inches). A key feature of this species is that its tail is shorter than the length of its head and body combined.
The Roof Rat (Rattus rattus) is a more slender species known as the black rat or ship rat. This rodent is considerably smaller and lighter, generally weighing between 150 and 250 grams (5.3 to 8.8 ounces). Maximum weight for large individuals is about 340 grams (12 ounces).
The Roof Rat’s body measures about 15 to 20 centimeters (6 to 8 inches) in length. Its most noticeable feature is its tail, which is consistently longer than its head and body combined, sometimes adding 19 to 25 centimeters (7.5 to 10 inches) to its total length.
Factors That Influence a Rat’s Growth
The size of any individual rat is determined by a combination of biological and environmental factors. Genetics plays a foundational role, setting the maximum potential size an animal can achieve. A rat inherits a genetic blueprint that predisposes it to a specific size range, meaning some individuals naturally possess the potential to be larger than others within the same species.
Resource availability modifies this genetic potential, with access to high-quality nutrition being a primary determinant of growth. Rats with a plentiful, consistent, and diverse diet, particularly one rich in protein, are more likely to achieve their maximum size. Conversely, rats in overcrowded areas or those facing intense competition for food may experience stunted growth.
Age is another significant factor because rats continue to grow throughout their lives, although the growth rate slows substantially after they reach sexual maturity. Older rats, which have survived longer in the wild, are often the largest specimens encountered. Also, sexual dimorphism exists; male Norway rats, for instance, generally grow larger and heavier than females due to hormonal differences that influence their growth rates.
Size Extremes and Misidentification
Reports of truly gigantic rats often involve a mix of exceptionally large individual specimens and misidentification with other rodents. While the average adult Norway Rat weighs around 400 grams, an unusually large, well-fed individual may weigh up to 0.9 kilograms (2 pounds). This weight represents the high end of the scale for Rattus species commonly found in urban environments.
The belief that common rats can reach the size of a housecat is usually a case of mistaking one species for another. Several much larger, less common rodents are frequently misidentified as oversized rats. The Gambian Pouched Rat (Cricetomys gambianus), for example, is a large African rodent that can weigh over 1.3 kilograms (3 pounds) and reach a total length of 88 centimeters (over 34 inches).
Other large rodents, such as Muskrats (Ondatra zibethica), which can weigh between 0.9 and 1.8 kilograms (2 to 4 pounds), are also mistaken for giant rats. Their physical resemblance to a rat, combined with their substantial size, leads to exaggerated claims. While individual common rats can be larger than average, verified specimens rarely exceed 2 pounds, placing a limit on the size of the familiar urban pest.