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Rat Size: How Big Can Rats Get?

Rats are hairy creatures with long tails and a voracious appetite to match their ever-growing teeth. They can be found practically anywhere on the planet, especially in human-populated areas. If people dwell in a region, rats, particularly black rats, are almost certain to be present. 

They are rodents of medium size, differ from mice in the size, they are larger than them, but smaller than beavers. There’s a lot more to them than their dewy eyes and habit of littering. 

We’ll discuss how big these rodents grow and the average sizes of infant and adult rats in this article. We’ll also find out how long it takes for a rat to reach full size.

Rats Background 

Rats are members of the Rodentia order and share their whiskered appearance with mice, squirrels, hamsters, marsupials, and even gerbils. Rat tails, which are hairless and scaly, are one of the distinguishing features between rats and mice. 

These rodents have two large ears, black eyes, and long whiskers. Their coats are black, brown, gray, or white in color. Mischief is an apt term for a gang of rats.

There are around sixty different rat species, each having its unique color, diet, habitat, and size. The brown rat and the black rat are the two most common species. 

These animals have been domesticated and can now be kept as pets. The Norway rat (or common rat) is a brown rat, while black rats are sometimes referred to as roof rats (or house rats).

Types of Rats

A domesticated Rattus norvegicus, often known as a brown rat or Norway rat, is commonly kept as a pet in the United States. These rodents are available in a variety of hues, including Siamese, blue, cinnamon, variegated, and silver lilac. Albinos are white rats with pink eyes. Hairless and tailless rats are also available.

Black rats, wood rats, pack rats, and a variety of other species are examples of undomesticated rats. Rattus rattus, the Norway rat, and Rattus rattus, the black rat, are two of the most prevalent wild rats. 

Average Size of Baby Rat

These rodents, like the majority of them, give birth to their offspring directly. Female rats have a gestation period of 21 to 23 days, after which they give birth to young of them. These infants, sometimes known as pups or kittens, are about 2 to 4 inches long and weigh 5 to 8 grams.

Typically, a litter of rat kittens has 6-8 rat puppies. These newborns are typically born deaf, blind, and hairless. Their ear canals will begin to open up by the time they are two to three days old. 

Their eyes will open at about 7 days, and fur will begin to appear around 14-day-olds. Then they’ll start to gain weight and get bigger, and they’ll graduate from milk to solid food.

Average Size of An Adult Rat

Adult rats have tails that are 7 to 9 inches long and are between 9 and 11 inches long. Female adults weigh between 0.55 and 0.66 pounds (300 and 500 grams), and male adults weigh between 0.66 and 1.1 pounds (300 and 500 grams) (250 and 300 grams).

Depending on where they reside, adult rats can weigh more or less. Domesticated rats are known to have easier access to food and may become more robust as a result of a lack of strenuous activities. 

Wild rats, on the other hand, must scavenge for food and survive on whatever they can find. As a result, they are typically thinner and live shorter lives than housed rats.

The World’s Biggest Rat

According to the Smithsonian Institution, the Bosavi woolly rat is one of the world’s largest rats. In the crater of Mount Bosavi, an extinct volcano in Papua New Guinea, a Smithsonian researcher discovered this new species of enormous rat in 2009. 

It weighed about 3.5 pounds and was 32 inches in length, including the tail. This enormous rat had a thick silver-gray coat and showed no fear of humans. According to BBC News, this species may only exist within the confines of this volcano.

Average Rat Lifetime To Grow To Its Full-Size

These rodents have a life expectancy ranging from 12 months to 3 years. Black rats have been known to live for one year, while brown rats have been known to live for almost two years. However, they are domesticated rather than wild, the chances are significantly better.

At three weeks, puppies are usually weaned and left to fend for themselves. The sexual maturity of most rodents, including rats, begins at five weeks. 

Between the fifth and eighth weeks, females are usually ready to begin mating and reproducing. Males mature around the 10th to 12th week, so they are slower. Females can conceive within 48 hours of birthing pups, and gestation lasts between 21 and 23 days.

It’s critical to understand the difference between sexual maturity and physical growth. Even if they have had a litter before, most female rats reach full maturity at the age of seven months. 

At 8 to 9 months, male rats may stop growing. If they gain weight after these ages, it could be a sign of disease or overeating.

Are Rats Good Pets

Rats make excellent pets. However, not all rat species are suitable for keeping as pets, which is where fancy rats come in. 

Fancy rats are domesticated and bred as pets from common rats such as brown or black rats. In truth, the physical characteristics of pet rats and wild rats differ slightly.

Because they are fed directly by humans, pet rats are friendlier and have a more robust appearance. They also come in a wider range of coat colors. 

They come in a variety of colors, including soft gray, powder blue, beige, and even several tones. Fancy rats make good company and have been known to teach their owners tricks to keep them entertained!


We’ve spoken about a rat’s average lifespan and size in this article. If you’re on the fence about getting a rat as a pet, we hope this article has helped. 

But, if you are dealing with rodents in your home, you might also consider hiring professionals.

Please let us know about your thoughts and experiences! 


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