How Big do Pet Rats Get? (And How Long it Takes to Grow)

If you are planning on getting a pet rat you might be wonder just how big do they actually grow and if they are the right pet for you. Pet rats are larger than most other rodent pets, and rats have grown in popularity but one persistent question remains.

How big do pet rats get? The average size of an adult rat’s body is between 9 and 11 inches, not including the additional 7 to 9 inches for their tail, and they weigh approximately one pound (varying slightly depending on sex). A pet rat will grow from a pup/kitten to a full-grown adult rat in 6 months generally.

If you are wanting to buy a pet rat but you just don’t know if they are the right size for you, have no fear because this article will give you all the information you need on all things rat size.

How Large Do Pet Rats Get?

The most common species of the tamed rat to keep as a pet is the fancy rat, a name which originates from the meaning “to fancy” or appreciate and encourage the domestication of animals.

The fancy rat is the domesticated brown rat (Rattus norvegicus domestica) stemming from Norway, which can grow to be a whopping 2.0 lbs, but this would be a pretty rare occurrence.

Fancy rats possess a smaller frame than their wild counterparts but happen to sport larger ears and longer tails. It is generally accepted that domestic fancy rats are notably more compact and display honed facial features when compared to their wild brethren.

Mature male rats are often larger and bulkier than their adult female counterparts who tend to be smaller and have a slimmer frame.

Male fancy rats typically grow to be 1.0 to 1.4 lbs, whereas females are slightly smaller, weighing in at only 0.7 to 1.0 lb. Females are smaller and daintier than males, who are often noticeably bulkier.

How Long for a Pet Rat to Reach Maturity?

Pet rats will have fully matured in four to six months, though they may continue to grow in size for a little while longer. Don’t be fooled by this lengthier growth stage, male rats generally reach sexual maturity around 10 to 12 weeks of age, whereas females may go into heat (or estrus) as early as 8 to 9 weeks! It’s worth noting rats can reach sexual maturity at 5 weeks of age.

Most rats will be fully grown by seven months. Noticeable growth thereafter is more likely to be unhealthy weight gain than true growth spurts.

When a rat is born it weighs no more than a small coin, roughly 0.2 to 0.3 of an ounce, and is no more than 1.5 inches long! Their eyes will be closed and they will be furless, though their coat begins to rapidly fill in during the following days.

After approximately 14 days, their eyes will open and they may begin to explore.

By 6 weeks, the rats are fully weaned, their fur coat has grown in fully, and they can be adopted into a new home if desired. Ensure same-sex pairs to avoid unwanted litters!

How Much Space Does a Pet Rat Need?

Keeping in mind that rats should be adopted and housed, at the very least, in pairs, the smallest cage one should consider buying would be 29 x 14 x 17 inches or so.

It’s a good idea to get as big of a cage as possible; the more room available to the rats will encourage proper exercise and there will be plenty of room for mentally enriching mentally enriching toys. A general rule of thumb is to ensure there are 2.5 cubic feet of cage per pet rat, you can use this cage calculator here from the fancyratforum.

Be careful to avoid buying a cage with bars any farther apart than roughly 0.5 of an inch. Pet rats are at risk of getting their heads stuck or of being able to escape completely!

Mature males may get away with being housed in a cage with 1-inch spacing between the bars, however, it might be worth getting some mesh or chicken wire to fit around the cage just to be on the safe side.

Keeping a Rat at a Healthy Weight

Rats have voracious appetites that are seemingly open to anything remotely edible. It’s important to monitor their diet and ensure they aren’t overeating and packing on unnecessary weight.

Here are some quick tips to keep a rat’s weight in check:

  • Measure out their daily food portions instead of simply guessing at the appropriate serving sizes.
  • Look for a high fiber pellet variety.
  • Supplement each meal serving with a tablespoon of fresh, green vegetables.
  • Avoid feeding seeds, especially medleys as the rat will pick and choose based on flavor and end up missing out on nutrition; the higher fat content of seeds can also be unhealthy for rats.
  • Don’t feed pet rats junk food!
  • Weigh rats weekly to keep a close eye on their weight.
  • Provide rats with a method of exercise (cage wheels at least 11inches in width and a solid base, not wire), or encourage them to explore outside their cage.

What is the Biggest Rat in the World?

The Gambian Pouched Rat (African Giant Pouched Rat)

The Gambian pouched rat is the largest in the world, about the size of a small cat, exhibiting a nearly 3-foot length from tip to tail and weighing in at an astonishing 3 lbs! Its large size has drawn people’s attention and has made it a popular pet. The Gambian pouched rat is three times larger than a domesticated brown rat!

Their large size has drawn the attention of rat fanciers around the world. Unfortunately, the Gambian pouched rat is illegal to own in the United States, and traversing related legalities of ownership in Canada isn’t much better. According to breeders, obtaining a Gambian pouched rat is next to impossible.

Interestingly, this hefty rodent was put to work back home in Africa. A Belgian nonprofit found that these rats were superior to both humans and dogs when it came to sniffing out landmines. Better yet, while they’re a truly massive rat, they’re still too light to set off landmines, which means locating TNT has become much safer.

Thanks to their distinguished sense of smell, de-miners can mark the site and excavate safely. Should a mine be located, it is then detonated immediately.

The Bosavi Woolly Rat

Recently discovered in 2009 in the rainforest of Papua New Guinea, the Bosavi woolly rat (currently awaiting a scientific name) exhibiting a 2.69-foot length from tip to tail.

This behemoth of a rodent weighs in at around 3.3 lbs! This rat possesses a long, dense coat of gray-brown fur. The specimen that was captured showed no sign of being frightened by humans!

What is the Smallest Rat in the World?

A strikingly small variety is Osgood’s Vietnamese rat (Rattus osgoodi), found in the Central Highlands region of southern Vietnam, which is only 5 to 7 inches long! This variety of rat can only be found in Lâm Đồng Province!

Are Rats Bigger Than Mice?

Even though domesticated rats are smaller than their wild relatives, they are still a great deal larger than mice. The average domestic mouse can only grow between 5 and 7 inches long from their head to their tail, weighing in at a measly 1 to 2 ounces!

How to Spot the Difference Between a Baby Rat and a Mouse

A baby rat (also known as a pup or kitten) might be of a similar size to that of a mouse, but there are still ways to tell them apart. Mice have larger ears and their tails are longer in comparison to their body to that of a rat.

Young rats will have larger heads and feet when compared to the body size of the mouse. Finally, mice are a light gray or brown, with cream-colored (or lighter shade) stomachs.

Related Questions

Why Do Rats Have Such Long Tails? Most rats’ tails will be nearly as long as their bodies, a characteristic that sets rats apart from other rodents. The tail is an important component of rat anatomy as it serves three main functions:

  1. : The tail helps the rat to balance better than other rodents who are lacking in this department. A rat can use their tail to maintain some form of grip on their environment, regardless of where or what they’re walking on.
  2. : A rat’s tail appears fairly bald (aside from some peach fuzz), but this trait has a purpose: to help regulate their body temperature. A tail that’s too fuzzy may result in overheating; having barer skin surface area available will help to dissipate heat more efficiently. Alternatively, a lengthy tail may also be wrapped around a cold body to help warm up.
  3. : Rats express emotions through various movements of their tail. For instance, it’s common to witness rats wiggling their tails when they’re feeling exceptionally excited.

How Many Babies Do Rats Have Per Litter? Each litter contains 10 to 12 babies, and a female rat is capable of producing a litter every 3 to 4 weeks! That’s between 170 to 124 new rats per year!

Darren Black

I'm Darren Black, the owner, and author of I am from Scotland, United Kingdom and passionate about sharing useful information and tips about properly caring for an animal's wellbeing.

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