Most gerbils reach their full size by around 6 months of age. But some may continue to grow until they are a year old. This depends mostly on genetics and diet.

At what age are gerbils fully grown?

These animals reach adulthood at about three or four months, at which point they will be adult-sized and shaped, with a full body of fur and with the less-rounded heads that adults generally possess.

How big does a full grown gerbil get?

Mature gerbils weigh between 2 and 4 ounces, with males larger than females. According to the American Gerbil Society, that's about halfway between a mouse and rat in size. Grown gerbils are about 4 inches long, not including the tail.