Just How Big Are Rats? Rat Size Comparison

by Morgan Mulac

Just How Big Are Rats? Rat Size Comparison


Rats are absolutely fascinating creatures. Having human-like intelligence and curiosity, they can be trained to do many things. In addition, they come in various sizes and colors, ranging from the common Norway rat which can reach up to 16 inches in length to the tiny black rat that only grows to 3-4 inches.

During the course of this blog, we will explore the different sizes of rats, what determines their size, and how this affects the care they receive.

Understanding Rat Sizes: An Overview

If you understand how big your rat can grow as they mature, you can adapt the cage and toys to their growing size as well. While smaller rats can get through cage bars, you should ideally be buying your cage to accommodate a fully grown rat.

There are many factors that contribute to a rat's size, such as their diet, environment, species or breeding variations, and to many owners' surprise, the rat's sex.

Diet primarily impacts weight over anything else, which is why the proper rat food is vital, as it shouldn’t fluctuate too much from the average weight of its species.

A house rat's environment also impacts their size compared to wild rats. As pet rats have no predators, a longer lifespan, open access to a wide range of food, and don’t get as much exercise as their wild counterparts, they have more opportunities to grow to their full size. Many experts believe if a wild rat's lifespan was the same as a domesticated one, that wild rats would grow to a similar size.

Now let's take a look at the difference between species of rats and the breeding variations. All domestic rats are the “Fancy Rat” breed, save for the Gambian Pouched Rat. Overall, most fancy rats have a similar size and weight, but sometimes they are bred to be tailless, which can affect their overall body size.

Lastly, a rat's sex can greatly influence their body size. Females, regardless of the species of rat, will typically weigh less than their male counterparts. A female rat weighs 350 to 450 grams, while a male rat will weigh anywhere from 450 to 650 grams.

Tiny rat on a human hand

Size Comparison of Common Pet Rat Species

Below is a quick look at the size differences between a few different breeds and species of rats. While we will take a closer look at each rat on this list below, this is a handy guide to go off of.

Rat Breed/Species Average Body Length (without tail) Average Tail Length Average Weight
Fancy Rat 9-11 inches 7-9 inches 0.75-1.5 pounds
Dumbo Rat 9-11 inches 7-9 inches 0.75-1.5 pounds
Hairless Rat 9-11 inches 7-9 inches 0.75-1.5 pounds
Rex Rat 9-11 inches 7-9 inches 0.75-1.5 pounds
Gambian Pouched Rat 15-17 inches 14-17 inches 3-4 pounds
Brown Rat (Wild) 9-10 inches 7-9 inches 0.5-1 pound
Black Rat (Wild) 6-8 inches 6-8 inches 0.25-0.5 pounds

Fancy Rats

White rat on a cage

As mentioned above, Fancy Rats refer to almost all domesticated rats. For the sake of simplicity, we will be going with a standard fancy rat that has no specific breeding variations.

Fancy rats are part of the rodent genera and are the domesticated version of the wild brown rat. They come in a variety of colors, which are typically agouti coloring – which means three tones of color on the same hair follicle – these colors can range from light brown, all the way to dark chestnut. However, they are also available in solid colors like black, beige, and chocolate.

The agouti coloring is typically associated with the types of rats found in sewer pipes, but rest assured fancy rats are incredibly clean animals that are almost always free of common diseases associated with sewer rats.

Most fancy rats, regardless of breeding variations, will have the common naked tail that rats are known for. Which measures around 7-9 inches. Rat tails often serve a balancing purpose for rats, which is why it’s vital to adapt the cage to prevent tail injuries.

Dumbo Rats

Rat on a hammock

With the exception of their ears, dumbo rats are similar in size to most fancy rats. Their ears are placed on the side of their heads and are rounder than most fancy rats.

One care consideration if your dumbo rat has two dumbo rat parents is eye and jaw size. The genetics of two rats with the DMBO gene can affect these factors during the gestation period.

The good news is that typically breeders will breed a dumbo rat with a fancy rat that has the recessive DMBO gene to ensure that the shrinkage of eyes and jaw won’t occur or impact your rats. However, if your rat does have two dumbo parents, it is wise to ask your vet if any adaptations need to be made in their specific case.

Rex Rats

Rex rats are another breeding variation that is part of the fancy rat family. These rats are known for their curly fur patterns. Like human hair, this curl pattern can vary from a light wave to an incredibly coily

As far as health is concerned, it is important to keep in mind when it comes to Rex rats that they have a tendency for eye problems as their eyelashes will either have the same curly pattern or be totally absent.

Hairless Rats

Hairless rats are the last breeding variation we’re going to talk about in this blog. While their lack of fur makes them appear smaller than other fancy rats, it doesn't cause much variation in their size.

Similarly to rex rats, the absence of eyelashes on hairless rats leaves them more prone to eye problems like infections. There are not any adjustments you have to make, but keeping an eye out for any symptoms and contacting your vet is essential.

Hairless rat

Size Comparison of Common Wild Rat Species

Now that we’ve covered some of the most popular pet rats, let's take a look at the sizes of wild rat populations. It’s important to note here that all the rats below are considered invasive species and not intended to be kept as pets, while we understand the instinct to care for our ratty friends, this also includes not leaving pet food out for them.

Brown Rats (Brown Norway Rats)

The brown rat is probably what you picture when you think of a sewer rat. Often, these rats are very used to living in areas where there is human habitation and this is one of the reasons why they are so prevalent in big cities. Other names for Norway rats include wharf rat, common rat, and Hanover rat.

Overall, their appearance is very similar to standard fancy rats, as they usually have an agouti coloring. However, their body size is much smaller than their domesticated counterparts this is typically due to a lack of sources of food to choose from. These types are known to cause disease in humans, so it is recommended to stay away from them.

Black Rats (Roof Rats)

Black Rats, also known as roof rats or house rats, are another type of wild rat you may encounter. They’re slightly smaller than brown rats, and despite their name can present a few different color varieties, including light brown.

These rats were once thought to be responsible for the bubonic plague, but in more modern times are known to be an invasive species in many different places, as they can wipe out entire species of wildlife due to the diseases they carry.

Marsh Rats

Marsh Rice Rats are a type of rat native to North America and commonly found in the southeastern United States. These rats are typically much smaller than either brown or black rats and have grayish-brown or red fur depending on their location. This type of rat is just one of the many classified in the marsh rat family.

These types of rats have strong reactions to periodontal disease, and many scientists have proposed using them as a tool for studies of the long-term effects of the disease in humans.

Pack Rats

Pack rats are another species of rat native to North America, living in the western United States, Canada, and northern Mexico. Several species of pack rats fall under the pack rat family, but they are typically around 9.5 oz, and look similar to most rats. However, some pack rats have bushy tails.

It is well known that pack rats are nest builders and will often take whatever they can find to fill their nests with. Often, they are known to construct their nests in places that are unfavorable for humans, like the inside of house walls or inside the engine of a car. This can often be costly as these rats and their nests can damage wiring and more.

The Giants Among Rats: A Look at Exceptionally Large Rat Species

The vast majority of people are familiar with the smaller sizes of rats, but we want to take a look at some of the most massive rats that can be found around the world

The Bosavi Woolly Rat is one of the largest rat species in the world and is native to the Papua New Guinea rainforest. The Bosavi Woolly Rat can reach lengths of up to 32 inches and weights of up to 3.5 pounds. This makes them almost 3 times the size of the more common black and brown rats.

The Gambian Pouched Rat is another large rat species, native to the African continent. This rat can reach lengths of up to 35 inches and averages a weight of 3 pounds, but can go up to 9! This makes them about three times the size of a regular black or brown rat and the largest known species of rat in the world. In terms of their appearance, they are known for stuffing food in their cheek pouches and their extremely long tails. They were introduced to Florida as pets, but the importation has been banned since 2003.

Rat Size and Cage Requirements

It can not be overstated enough that a pet rat's size should be taken into consideration when choosing a rat cage. What many people don’t know is that rats have the ability to compress their skeletons to fit through extremely narrow gaps, so making sure your cage bars are small enough that their bodies cannot fit through is vital. The best bar spacings for rats are 1/2".

It is also important to get the right rat supplies to grow with your rat as they get bigger.

Small rat on a hand with wool gloves

Embracing Rats: Join the Adventure Today

Rats vary widely in size, from the small black and brown rat to the large Bosavi Woolly Rat and Gambian Pouched Rat. When considering a pet rat, you should make sure to select a cage with the appropriate bar spacing to ensure the safety of both your rat and your home. With the right rat care, rats can make wonderful companions and offer hours of entertainment.


How big is a common rat?

The common rat species weighs around 0.31 – 1.1 lbs. This is only a tiny bit smaller than domesticated Fancy Rats.

How do I know if I have rats or mice?

It is important to note that the primary difference between rats and mice lies in their size. A rat has a longer and heavier body than a mouse does. Adult mice are 3-4 inches long, and rats are 9-11 inches long.

What does a wild rat look like?

Wild rats are typically brown or black in color and have a long, scaly tail with a pointed muzzle. They have a stocky build and weigh up to 11 ounces. They are usually around 7-9 inches in length excluding their tail. They have large ears and beady eyes, and their fur tends to be coarse and shaggy.

How big do city rats get?

Some cities have Rats that can reach up to 2 pounds in size if given access to an ample food source, despite the fact that the typical size of this species is 0.31-1.1 lbs.


Have Questions About Rats?

Email us at cages@qualitycage.com

Author Bio: Morgan Mulac

Morgan Mulac has been working as a freelance writer for five years and has developed a passion for exotic pets. Dedicated to learning about exotic animals from all over the world, she seeks ways to share her knowledge with new owners about how to better care for their animals. If Morgan is not researching or writing about exotic pets, you can find her enjoying a cup of coffee and planning her next adventure. https://morganmulac.com/

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