How large can a rat grow? - briefly
The largest documented members of the common Norway rat can attain approximately 1 kg in weight and a total length near 50 cm, tail included. Typical domestic forms average 200–300 g and a body length of 20–25 cm.
How large can a rat grow? - in detail
Rats can reach a considerable size, but the maximum dimensions vary among species and individual conditions. The most common laboratory and urban species, the brown (Norwegian) rat, typically attains a head‑body length of 20–25 cm and a tail of similar length, with adult weights ranging from 300 g to 500 g. In exceptional cases, well‑fed individuals have been recorded at 700 g, with total length approaching 45 cm.
The roof rat, another widespread species, is slightly smaller, averaging 18–22 cm in body length and 250–400 g in weight. Both species share a rapid growth phase: newborns weigh about 5 g, double their mass within the first week, and reach sexual maturity at 8–10 weeks, when they approach 80–120 g.
Wild giant rats illustrate the upper limits of rodent size. The Bosavi giant rat (Papagomys theodorverhoeveni), discovered in Papua New Guinea, measures up to 80 cm in total length, including a 30 cm tail, and can weigh as much as 1.5 kg. Captive records for the brown rat list a maximum weight of 1.2 kg and a total length of 50 cm, achieved under optimal nutrition and low stress.
Factors that influence ultimate size include:
- Genetic potential of the species or strain.
- Availability of high‑calorie, protein‑rich diet.
- Absence of disease and parasites.
- Environmental temperature, which can affect metabolic rate.
Growth follows a predictable pattern: the first month accounts for roughly 60 % of adult body mass, the second month adds another 20 %, and the remaining increase occurs over the next two to three months. After sexual maturity, weight stabilizes unless additional caloric surplus is provided.
Compared with other rodents, rats rank among the larger members but remain smaller than capybaras (up to 65 kg) and beavers (up to 30 kg). The documented extremes demonstrate that, under favorable conditions, a rat can exceed one kilogram in weight and approach half a meter in total length, representing the greatest size observed for this genus.