How Big Do Small Rats Get?

The typical size of a small rat varies widely, but the average adult weighs between a half-ounce and two pounds. They are smaller than mice, but larger than many other rat species. The largest species of rat in the world, the Sumatran Bamboo rat, can reach twenty inches in length. Although they may look small in appearance, these rodents can grow up to twice their average size in a few short years.

There are several factors that influence a rat’s size. One is the environment it lives in. Rats living in small, unoccupied cages are generally smaller than larger ones. The world’s largest rat, the Bosavi woolly rat, was discovered in the rainforest of Papua New Guinea in 2009. The Bosavi woolly rat is the biggest species in the world, weighing around 3.3 pounds (1.5 kilograms). Another species is the Osgood’s Vietnamese rat, which grows to between five and seven inches (12 and 17 centimeters) long.

Rats grow at a rate of about one inch per month for the first three months. Then, their growth slows until they are fully mature. This maturity is different from sexual maturity, which is important to know if you plan on keeping more than one rat.