Before you get a rat, you should learn about how big these animals can get. This is especially important if you are buying a baby rat.

This information will help you take good care of your new pet. We will go over all the facts you need to know about the size and development of these animals.

Average Rat Length

The average length of a rat’s body is anywhere from 9 to 11 inches once it is fully matured. This is not including the tail, which can be up to nine inches long. This means that your rat gets as large as 20 inches overall as an adult.

Rats have the ability to compress their body to fit in tight spaces, which is something that serves them extremely well in the wild. This allows them to escape from all sorts of predators on a daily basis. It is also what allows them to get into people’s homes in both rural and urban areas. Even a tiny opening in your home could be enough for a rat to get through.

Dumbo Rats

There is a certain type of rat called the “dumbo rat”, which is a reference to their larger-than-normal ears. When a rat gets two copies of a certain gene from its parents, its body develops into a uniquely round overall shape. While these rats are fairly rare, they are still probably more common than you’d initially think.

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Dumbo rats are still brown rats; they just have a genetic mutation that makes them look different. Brown rats that do not have this mutation are commonly referred to as “top-eared” rats. A dumbo rat is usually the same size as most others (around 20 inches). The only difference is that their ears are noticeably larger.

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Physical Development of Pet Rats

When a rat is born, it only weighs approximately six grams and measures 1-2 inches. These animals are born with bare skin with no trace of fur whatsoever, and they are completely blind. Once they are a bit older, their eyes start to open.

The average rat will be able to see clearly by around two weeks old. They will also start moving around more and become fairly active.

Once a rat reaches six months old, it is considered a fully matured adult. This is the point at which they reach their full size, which is around 20 inches long.

Rat Gender and Size

In general, male rats tend to be larger than females as a whole. Females are almost always slimmer and shorter than their male counterparts.

Interestingly enough, there are actually personality differences between male and female rats. Males are typically more receptive to cuddling and being handled. Female rats are usually a bit more standoffish and timid, but they are far more active than males.

If you want a rat that is fairly small and moves around a lot, you’ll probably want to get a female. Males are a good choice for those who want to handle their rat for long periods without agitating their pet.

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Choosing a Cage

A vast majority of rats need a cage that offers at least two cubic feet of space. If you are going to keep two rats together (which is recommended), you’ll need an even bigger cage. It should measure 30 by 18 by 36 inches minimum. Keep in mind that this is for just one rat.

Both male and female rats need about the same size cage. While males are bigger, females are significantly more active. It is very important that your pet has plenty of space to run around in. The fact is that keeping your rat in a cage that is too small is not good for them, both mentally and physically.

 If you are keeping a baby rat in a small cage, you will need to get a bigger one as it grows. It is recommended that you get a cage with the dimensions mentioned above, regardless of the rat’s current age. This way you won’t have to worry about having to buy a larger one later on.

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Feeding Your Rat

As your rat grows and gets bigger, you will need to feed it appropriately. A baby rat should be given lactated ringers solution for the first 24 hours of its life.

It is extremely important that your rat gets enough to eat based on its age and size. If you do not give your rat enough to eat while it is still growing, it can develop a number of very serious health problems.

Largest Rat Species

There are a number of different rat species, but most of them reach the same size as adults. The Bosavi Wooly rat, however, is significantly larger than the rest. This particular species can grow up to 32 inches, which is the world record. This rat weighed a whopping 3.5 pounds.

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The Bosavi Wooly rat was first discovered in 2009 by a biologist who worked for the Smithsonian. It is very likely that all of these rats live inside a particular extinct volcano in Papua New Guinea.


  • The body of an adult rate measures 9 to 11 inches, while the tail alone can grow to nine inches.
  • The average overall length of a fully matured rat is 20 inches.
  • Dumbo rats are regular brown rats, except they have a genetic mutation that results in abnormally large ears.
  • A baby rat measures only 1-2 inches when it is born.
  • A rat’s eyes do not open up until they are about two weeks old.
  • Male rats tend to be noticeably heavier and larger overall than females.
  • Female rats are usually more active than males, but they are also more timid.
  • Your rat’s cage should offer at least two cubic feet of space.
  • It is crucial that you feed your rat the appropriate amount as it grows.
  • The largest rat species is the Bosavi Wooly, which was discovered in an extinct volcano in Papa New Guinea.
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Hi! I'm Anna and I´m a certified cynologist (KAU, ACW). Expert, blue cross volunteer, owner of Chinese crested kennel "Salvador Dali" and breedless friend called Fenya. "I can't imagine my life without dogs and I totally support the idea #AdoptDontShop".