How Big Can a Rat Get?

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Rats grow to be much larger than mice, and their tails can be nearly as long as their bodies. Rats include a great number of both domesticated and wild species. The rat's species affects the ultimate size of a rat.

Certain species of cloud rats, native to the Philippines, can reach more than 4 pounds, according to the BBC Earth News.


Male rats tend to become heavier and bigger than females. Rats have been domesticated for more than 100 years, according to the American Fancy Rat and Mouse Association. A baby rat you adopt as a pet might grow to several times its original size. A rat's smaller ears, wider muzzle and thicker tail help to distinguish it from a mouse. A well-fed rat becomes larger than a rat with a limited food supply.


The American fancy rat, commonly kept as a pet, is a domesticated Rattus norvegicus, also called a brown rat or Norway rat. Some of these rats come in special colors such as Siamese, blue, cinnamon, variegated and silver lilac. Pink-eyed white rats are albinos. There are also hairless and tailless rats.

Undomesticated rats include black rats, wood rats, pack rats and numerous other species. The Norway rat and the black rat, Rattus rattus, are among the most common wild rats.


Domesticated rats provide tame companionship, and some perform in movies.
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The average body length of adult rats is 9 to 11 inches, plus a tail of 7 to 9 inches, according to the American Fancy Rat and Mouse Association. Your pet rat could grow to be up to 20 inches long from nose tip to tail tip. In rare instances, an adult male pet rat might grow to weigh 2 lbs.

The rat's body can compress to allow it to access small spaces -- an opening the size of a quarter in your wall or near a pipe will allow a rat to enter your home. If the rat can get its head through an opening, it can slither inside.

Biggest Rats

The Bosavi woolly rat is one of the world's largest rats, according to the Smithsonian Institution. A Smithsonian biologist discovered this new species of giant rat in 2009 in the crater of Mount Bosavi, an extinct volcano in Papua New Guinea. It weighed close to 3.5 lbs. and measured 32 inches long, including the tail. This gigantic rat has a thick silver-gray coat and was completely unafraid of humans. This species might live solely inside this volcano, according the the BBC News.



About the Author

Gryphon Adams began publishing in 1985. He contributed to the "San Francisco Chronicle" and "Dark Voices." Adams writes about a variety of topics, including teaching, floral design, landscaping and home furnishings. Adams is a certified health educator and a massage practitioner. He received his Master of Fine Arts at San Francisco State University.