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The Rex Guinea Pig: Essential Size & Care Guide

When you think of guinea pigs, you’re most likely picturing the cuddly and social animals from South America with adorable faces — one of the reasons why they’re such popular pets. But the Rex guinea pig (also known as Cavia Porcellus) — undoubtedly cute and cuddly — is a large exception. In fact, they are the biggest of the guinea pig breeds out there and up there when it comes to the most popular breed, too. 

Not to be confused with the Teddy guinea pig (which it often is), the Rex has a social and friendly temperament that any household would welcome. However, the size of the guinea pig you choose to buy is a significant factor to consider — after all, it determines the amount of food you need to buy and how big the cage should be

Adorable female ginger guinea pig.
Red guinea pigs ( Cavia Porcellus) are very popular small pets.

But how big do Rex guinea pigs get? This guide will answer any question you may have about the size of Rex guineas, from birth to adulthood and more. 

What is a Rex guinea pig?

Let’s begin with the basics — what exactly is a Rex guinea pig? The word ‘rex’ originates from the Latin word for ‘king.’ Just like the Tyrannosaurus was the ‘king of the dinosaurs’ (so to speak), the Rex guinea pig is the king of all guinea pigs.

It may be because they are so much larger than other guinea pig breeds. But the truth is that this breed gets its name from its most distinctive feature — its coat. When it comes to animal breeds, the word ‘rex’ typically refers to the genetic quality of the animal’s fur.

Rex guinea pigs have very short fur and lack guard hairs. This makes their coat seem more bushy and bristly than other kinds of guinea pigs that you might be used to. Despite its coarse appearance, the Rex coat is soft to the touch — some might even say they look like a fuzzy potato.

These little guys have coats in a variety of colors, anywhere from white, gold, brown, or red. And they have an average lifespan of around five to eight years, though some live longer.

So, if the Rex guinea pig is one of the largest breeds out there, how big do they get? Let’s have a look.

How big do Rex guinea pigs get?

guinea pig’s full size is dependent on a lot of things. These factors include the breed, first of all, but also their age, their sex, and other genetic factors. The same goes for the Rex.

There is a lot of variation in the size of different Rex guinea pigs. It depends on the animal’s genetic makeup, age, or sex. Even diet and lifestyle play a significant role in the size of your guinea pig.

Ginger guinea pig with a black hat.
The size of the guinea pigs depends on a lot of factors.

Generally, a full-grown Rex’s average size is around eight and 15 inches (20.3 – 38.1 cm) from nose to tail. In many cases, they can be as long as 18 inches (45.7 cm). It’s no wonder these gentle giant guinea pigs are one of the biggest breeds out there.

Of course, none of this makes these furry friends any less special.

Size at birth

Following a healthy pregnancy in guinea pigs, you can expect anything between one and eight pups to be born, although four is common. 

Baby guinea pigs, also called pups, are born around similar sizes, regardless of breed. Most of them are no longer than three or four inches (7.6 – 10.1 cm) and weigh around three and a half ounces (99.2 grams), which, as you can imagine, is relatively small. 

Within a short period over the next few weeks, the pups will grow substantially, even doubling in size after nine weeks. This is just an estimation, though. Rex pigs (and all guinea pigs, for that matter) can be born outside of the average estimates depending on multiple factors.

One factor is the number of pups. If the mama guinea pig is pregnant with more or fewer babies, their size can be impacted after birth. If she is pregnant with more babies, there will be less space overall in the womb and fewer nutrients, making the pups slightly smaller. Conversely, if there are fewer babies, they might be bigger.


After the initial growth spurt at around nine weeks of age, the growth of the guinea pigs slows down a bit. Around the three-month mark, the guinea pig will likely grow to about seven to nine inches (17.7 – 22.8 cm) and their average weight will be between eight and 12 ounces (226.7 – 340.1 grams). 

Of course, this varies slightly based on the animal. At six months of age, their length does not increase all that much. It grows somewhat, but not more than an inch or two. Their weight, however, can increase to between 17 and 25 ounces (481.9 – 708.7 grams). 


The Rex guinea pig reaches maturity at around 14 months. A fully-grown adult Rex gets anywhere between eight and 18 inches long (20.3 – 45.7 cm) and weighs between one and three pounds (0.45 – 1.36 kg) — I did say they’re the largest guinea pig breed! 

Male vs. female Rex guinea pig size

As mentioned previously, a guinea pig’s size depends on multiple factors — one of those being sex. The same goes for Rexes. Like most of the animal kingdom, even these adorable piggies are subject to the age-old idea that males are larger than females.

A beautiful guinea pig with some nature in the background.
Female Guinea pigs are a bit smaller than male Guinea pigs.

Male guinea pigs, called boars, are typically on the larger end of the range regarding their weight and length. Conversely, female guinea pigs, called sows, are slightly smaller. They both fall within the same ranges as mentioned above, but males are usually larger.

Cage size for Rex guinea pigs

Now that you know just how big a Rex guinea pig can get, you’ll need to learn more about how their size affects their environment and needs. Since they are so big, a large cage must be used to accommodate their size.

Guinea pigs are generally curious creatures — they like to explore all their environment offers, foraging and frolicking around to their heart’s content. And, as you’d expect, larger piggies need plenty of space to explore.

Since you should tend to keep guinea pigs in pairs (Rexes are no different), they won’t like to feel cramped. Can you imagine having two massive guinea pigs in a small cage? It’s not great. So, making sure the space is large enough to accommodate your little fluff balls is essential.

But what exactly makes up the perfect enclosure for your Rex guinea pig? 

Size of living and exercise space

The first part of any guinea pig enclosure is the living space. This is where they will spend most of their time — eating, sleeping, pooping — you know, all of those good things. Then, you have the exercise space, which, more often than not, is combined in the same enclosure.

Your little piggies will require several hours of exercise per day, whether that be in the form of foraging or roaming around their enclosure. The cage must be big enough to accommodate these activities and separate the play from the living space.

According to the Humane Society of the US, the absolute minimum size recommended for guinea pig housing for two is 7.5 square feet (0.69 square meters), or 30 by 36 inches (76.2 by 91.4 cm). However, a larger enclosure — up to at least 10.5 square feet (0.97 square meters) or 30 by 50 inches (76.2 by 127 cm) — is preferred. After all, you’ll want them to have plenty of room and not feel cramped in their home.

Two long hair guinea pigs are about to play with the wheel on their cage.
Exercise is very important for your piggy overall health.

But that is for guinea pigs in general — remember, the Rex is one of the largest of any guinea pig breed out there, so any bigger cage size is preferable. An open living habitat can be especially spacious.

Overall, the bigger, the better — so try to get a cage that is as big as your space can accommodate. If you’re up for it, a cage size of 11 square feet (one square meter) is fantastic.

Read more: The Minimum Cage Size for Two Guinea Pigs.

Rex guinea pig care guide

Now that you know more about the size of the Rex and its cage size requirements, there are some things you should keep in mind when caring for these gentle giants, which typically make low-maintenance pets. Here are some of the main takeaways to remember.

Rex guinea pig coat

The Rex’s coat is unique — after all, that is what they are named after. Their curly coat lacks guard hairs but is still quite soft. Don’t worry — just because their coat is special doesn’t mean you must go through some crazy and intricate grooming process.

All your little Rex needs is to be brushed on a regular basis (at least twice a week) with a bristle brush for guinea pigs that you can find online or at a pet store.

Rex guinea pig claws

Another thing you should know about the Rex is that their claws are pretty thick and grow faster than other breeds. Make sure to get a good pair of nail trimmers made for thicker nails, and keep them at a safe length. 

You definitely wouldn’t want your little piggy to hurt themselves or get their claws hooked on things, right?

Rex guinea pig diet

As a general rule, you’ll want to give your guinea pig a constant supply of fresh water and food, no matter the breed. They are vegetarians, so they need good-quality hay, pellets, and plenty of fresh, leafy greens to prevent health issues.

A cute guinea pig eating celery on a white background.
A healthy Guinea pig diet should include plenty of water and leafy greens.

Rex guinea pigs need the same nutrients and vitamins as other breeds but are particularly prone to Vitamin C deficiency, so make sure to give them plenty of fresh fruits and veggies or, in some cases, a piggy-safe supplement. But it’s always best to check in with your local vet if you aren’t sure.

Next Read: Fruits Guinea Pigs Can Eat

Rex guinea pig size | Wrapped up

Rex guinea pigs are undoubtedly one of the most popular guinea pig breeds around. Their sweet and loveable personalities are wrapped in a soft and cuddly package. And since they are one of the biggest breeds out there, you get their greatest qualities ten-fold.

One thing is for sure, though — this king of the guinea pig world is just what you need to bring that extra love into the family. As long as you keep in mind their size and what they need for their care, you’ll be golden.

Now that you know about the Rex guinea pig and its size, why not take a look at the American Guinea Pig Size guide? These piggies are the most popular short-hair guinea pig breed out there, and you will surely love them just as much as the Rex.

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