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How Big Do Rats Get: Pet Rats vs. Wild Rats

The smallest rat is about 5 inches long, and the largest reach up to 20 inches, without the tail. There are over 60 species of rats of different sizes. Common rats (brown and black rats) weigh about one pound and average 15 inches in length. Pet rats are about the same weight with slightly smaller bodies, averaging 10 inches in length.

Rats are medium-sized rodent mammals that are part of the genus Rattus, with a wide spread around the globe. 

The most common species of rats (as household pests) are Rattus rattus (black rat) and Rattus norvegicus (brown rat). 

Rats stand out from mice by their larger body volume, elongated snouts, and longer tails.

In this article, you will learn how big rats can get, how big baby and adult rats are, how much they weigh, and much more.

Rat Size     

The most common rats, brown rats, can reach up to 1.5 lbs. of body weight (700 g) and a length of 10 inches (25 cm) without the tail. Their tail is usually as long as their body length (11-24 cm). Pet rats are smaller than common rats.

Rats are among the most widespread rodents and live everywhere in the world except Antarctica. They are very adaptable, being able to live in various habitats (rainforests, swamps, fields, etc.).

Rat Size

Many factors determine how big a rat can get:

  • Species
  • Sex (males are larger than females)
  • Age (babies are smaller than adults)

There are over 60 species of rats. The smallest average size is about five inches and the largest rat measures about 20 inches.

Rat speciesRegionSize (without the tail)
Osgood’s ratVietnam5 inches
Tanezumi RatAsia8.25 inches
Red Spiny RatAsia8.26 inches
Bushy-Tailed Wood RatUSA and Canada8.7 inches
Lesser Bandicoot RatSouthern Asia9.85 inches
Brown RatEverywhere, except Antarctica11 inches                
Mountain Giant Sunda RatIndonesia and Malaysia10 – 12 inches
Northern Luzon Giant Cloud RatPhilippines15 inches
Bosavi Woolly RatPapua New Guinea16 inches
Gambian Pouched RatAfrica17 inches
Sumatran Bamboo RatChina and Sumatra20 inches

The brown rat is the most common rat species. It can reach a height of 2.4″-3.5″ (6-9 cm) and a body length of 6-10 inches (15-25 cm), without a tail. Its tail is shorter or the same length as the body, about 4.3-9.4 inches (11-24 cm).[2]

Black rats are also quite widespread and are slightly smaller than brown rats, with a maximum body length of eight inches (20 cm) and a tail of 10 inches (25 cm).[3]

Pet rats are about the same size as brown rats.

Related: Male vs. Female Rats

How Big Are Baby Rats?

Baby rats’ bodies measure about two inches long and weigh 6-8 grams when they are born.[4]

The size of common wild rat babies is similar to that of pet rat pups. They are born hairless and with their ears closed and have short tails. The ear canals open in 2-3 days, while the eyes open after seven days. The fur is visible from two weeks of age.

After weaning around the age of three weeks, baby rats switch to solid foods. They grow up to be four inches long by juvenile age.

How Big Are Baby Rats

How Big Are Adult Rats?

The average adult rat measures about 10-11 inches in length. The difference in size between wild and pet rats is not significant. Rats’ tails are almost as long as their bodies.

Males are larger than females and weigh 0.6-1 lbs. (270-450g). Pet rats can weigh even more, depending on how much they eat.

How Big Do Pet Rats Get?

Pet rats can get up to 1.4 lbs. body weight and a total length of 20 inches. They are considered subspecies of the common brown rat and reach about the same size in adulthood. Pups weigh as much as a nickel and are about two inches long when they are born.

Pet rats or fancy rats are small mammals. They are smaller than guinea pigs but bigger than many other rodents:

  • Mice
  • Gerbils
  • Degus

Pet rats weigh about 0.5-1.4 lbs. body weight as adults. There are cases when adult males can weigh up to 2 pounds (0.9 kilograms).[5]

They have an average body length of eight inches without a tail. The tail is almost as long as their body. The total average length of a pet rat is 20 inches.

Males are larger than females.

Related: Types of Pet Rats (Fancy Rats)

How Big Can Regular Rats Get?

Regular male rats can reach up to 11 inches in length and 1.1 lbs. in weight. Female rats are smaller. In rare cases, common brown and black rats can reach 2 lbs. Regular rats measure around 20 inches long from the tip of their nose to the end of their tail. The tail is as long as their bodies or smaller.

How Big Can Regular Rats Get

How Big Is the Average Rat?

The average common brown rat can reach up to 1.1 lbs. (500g). They can measure up to 10 inches long without the tail. The tail is usually as long as the brown rat’s body.

Rat Growth Chart

Rats weigh only a few grams when they are newborns. In about two months, they become young adults and weigh about 300 grams. When they become adults, some rats can weigh over 500 grams.

Rat ageBody weight (grams)[6]
0-21 days (neonatal)~14 grams (0.03 lbs.)
21-35 days (weanling)~45 grams (0.09 lbs.)
35-63 days (preadolescent)~115 grams (0.25 lbs.)
>63 days (young adult)~300 grams (0.66 lbs.)
Adult~500 grams or over (1.1 lbs.)

When Are Rats Fully Grown?

Rats are fully grown by the age of eight months. Females are fully grown at seven months, and males at nine months. After this age, rats stop growing in length. Pet rats live on average 2-3 years, while wild ones about a year.

Domestic rats are more likely to live longer because they are safe from danger, predators, and disease. Black rats live for about a year, and brown rats for two years.

The oldest pet rat, Rodney, has lived for seven years.

Facts about rats growth:

  • Pet rats can live up to three years. 
  • Kittens wean at the age of 21 days, whereafter they are left on their own.
  • Sexual maturity in rats begins at five weeks in females and 10-12 weeks in males.
  • Females are fully grown at seven months of age and males at 8-9 months.

Rats start to gain weight after they have reached their maximum size due to overeating or disease. If you have rats as pets, it is important to be careful with their nutrition, so they do not gain extra weight.

Related: Rat Lifespan: How Long Do Rats Live?

When Are Rats Fully Grown

How To Help Your Rat Grow To Its Maximum Size

Pet rats get more attention and care than wild rats, therefore they will live longer. The life expectancy of pet rats is 2-3 years.

Here are some tips to help your rat reach its maximum size:

  • Make sure it’s healthy. Pet rats are prone to respiratory infections and mammary tumors.
  • Take special care of females after giving birth. Females that have had puppies are even more prone to mammary cancer.
  • Separate the female from the male if she is pregnant. Females can become pregnant again two days after giving birth.
  • Feed your pet rat a varied, complete, and healthy diet, and do not use dusty bedding (they have very sensitive airways).
  • Contact your veterinarian if you think something is wrong with your pet rat.


How Big Is a Full-Grown Rat?

Adult rats reach 9-11 inches in length without a tail. The tail is 7-9 inches long. Males weigh 0.66-1.1 lbs. body weight and females reach 0.66 lbs. Adult rats can weigh more or less depending on where they live and whether they are domestic or wild.

How Big Is the Biggest Rat?

The largest rat is the Sumatran bamboo rat. It reaches up to 20 inches, excluding the tail. The tail is short, only eight inches. They are smaller than the pouched rat (Gambian rat) in length but are heavier. They weigh up to 8.8 lbs. The Sumatran bamboo rat is found in China and Sumatra and feeds on bamboo and sugarcane. They are considered harmful because they damage crops.

Do All Rats Get Big?

Not all rats get big. The smallest rat is about five inches long and lives in Vietnam, while the biggest rat can reach 20 inches long (China and Sumatra). Common rats live for about one year, and many of them do not reach full size due to the dangers they are exposed to. Pet rats can get fat if you overfeed them.

About Iulia Mihai (DVM)

Dr. Iulia is a certified veterinarian with more than 10 years of experience in the field. With extensive knowledge of diet, care, and medication, she helps Misfit Animals provide readers with accurate knowledge on technical topics.

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