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When Do Guinea Pigs Stop Growing? [How Big Do They Get?]

If you’ve never owned a guinea pig before and are thinking of getting one, you may have wondered how fast they can grow and what size they can eventually reach. Or you may already own a baby guinea pig that is rapidly growing and wonder when the growth rate will slow down. The answers to these questions may determine what size hutch size you purchase, how much food to buy or how many guinea pigs you end up adopting. It is therefore important to be aware of the various growth stages in a Guinea pigs’ life, so that you can properly support them as they mature.

So, when do guinea pigs stop growing? Guinea pigs generally stop growing at around the age of 14 months old. They typically reach 8-12 inches (20-30cm) in length and between 1.5-2.6 pounds (700-1200 grams) in weight when fully grown.

However, this is only a broad representation of a guinea pigs’ final size.

As you will discover in this article, there is not only variation between males (boars) and females (sows) but variation between different breeds, as well as many other factors that influence the final size of a guinea pig.

Let us now take a closer look at the typical size of your average domestic cavy, along with answering those similarly related questions that you may have.

We’ll also be looking at how you can ensure they reach their full size potential – if you do decide to own one!

How Big Is A Full Grown Guinea Pig?

Guinea Pigs grow much bigger than other pet rodents such as hamsters, mice and rats. By the time they have reached full maturity, a guinea pig can reach up to 12 inches in length and 2.6 pounds in weight.

However, this is usually the upper limit for most breeds and just because your cavy isn’t quite this big yet, that doesn’t necessarily mean they still have growing to do.

A guinea pigs’ final length can vary between males and females.

The length of a cavy is measured from the nose to the rear, and boars will often grow to be slightly longer than sows, at 10-14 inches and 8-10 inches respectively.

When it comes to weight, males also tend to be heavier than their female counterparts.

The average male will weigh between 2-2.6 pounds (900-1200 grams), while the average female will weigh between 1.5-2 pounds (700-900 grams).

However, it important to bear in mind that there can be significant variation between different breeds of guinea pig.

Although the majority of breeds are similar in size and fall within the ranges above, some breeds are much larger or smaller than this average.

We will go into this in more detail later.

At What Age Is A Guinea Pig Fully Grown?

A guinea pig is not considered to be an adult until 6 months of age, but they are not necessarily fully grown at this point.

Most guinea pigs will continue to grow until they are about 14 months old and as they get closer and closer to this age, their growth rate slows until eventually they are not growing a significant amount anymore.

It can be difficult to estimate a guinea pigs age if you haven’t owned them since they were a pup.

Size can be a good estimation of age in the absence of anything else. Using the guidelines below, you may be able to make a rough estimate of your guinea pigs age, but this is easier to do while the cavy is still young.

As mentioned, growth plateaus after full maturity so other factors such as general condition and health may be a better indicator for age.

Do Guinea Pigs Keep Growing?

Once guinea pigs reach full maturity at 14 months of age, they generally stay around the same size for most of their adult life. They can continue to grow a little bit past this, but not by a significant amount.

However up until this point, while still young, guinea pigs will grow rapidly.

Its good to have a rough idea of how much a guinea pig will weigh at different stages of their development, as this will allow you to ensure they are healthy and stay on track as they mature.

  • Baby guinea pigs, or pups, only weigh about 0.15-0.25 pounds and measure between 3-4 inches at birth – only a tiny fraction of their final adult size! Typically, a sow will give birth to 3 pups, but litters can contain up to 6 pups; larger litters will often result in smaller offspring.
  • The pups will grow very quickly from here, doubling in size in the first 8 weeks of life. The average guinea pig weight at this stage will be 0.4-0.55 pounds and the average length will be 6-8 inches.
  • Following this, growth rate will begin to slow down. By the time the pup is 16 weeks of age they will be about 8-10 inches in length and weigh around 0.8-1.1 pounds.

At this point most guinea pigs will be almost the same length as their final adult size, but they will still gradually increase in weight until they are fully mature.

It is not uncommon for a guinea pig to double in weight between 16 weeks and 14 months of age.

Are There Different Sizes Of Guinea Pigs?

There will always be some variation in size between guinea pigs of the same breed, due to several factors including genetics, nutrition and amount of physical activity.

And some of the differences between individuals will be due to overall body composition – some cavies will be fatter than others and may even be considered overweight.

But the biggest size variance between guinea pigs comes when comparing individuals of different breeds.

The American Cavy Breeders Association (ACBA) recognizes 13 different guinea pig breeds, and there are some others that are not yet recognized; these all can vary significantly in size.

Some breeds are much larger than the average guinea pig, such as the Rex guinea pig which can grow up to 17 inches at full maturity.

Other breeds, such as the Texel and the American guinea pig, are often slightly smaller than the average.

So, as you can see, the breed of cavy can have a big impact on the final size at maturity.

How To Ensure Your Guinea Pig Reaches Their Full Size Potential

There are many factors that influence the rate of growth of a guinea pig.

Some cannot be controlled, such as genetics. But other factors like nutrition and housing are under the owners’ control, and it’s important to know how to maximize on these to allow your guinea pig to be as healthy and happy as possible.

Let us now take a closer look at each one:

Diet

Perhaps the most important factor in ensuring healthy growth for your guinea pig is proper nutrition.

The cavy needs enough energy to support growth as well as essential vitamins and minerals.

Guinea pigs are herbivores, whose teeth are continuously growing. Chewing on plant material helps prevent their teeth from becoming too long, therefore preventing dental disease which may otherwise prevent them from eating enough.

They require a reliable supply of fresh grass or grass hay as well as leafy green vegetables such as broccoli, cabbage, lettuce and spinach.

Vitamin C is also essential for guinea pigs; they cannot make their own so must acquire this from their diet.

They need between 10-50mg per day, and whilst much of this can be attained from leafy greens, it’s worth supplementing with small amounts of citrus fruit to ensure this quota is met.

And of course, they need constant access to clean water.

It is important to remember that just because your guinea pig is big, that doesn’t always mean they are healthy.

If they are overfed or don’t get enough exercise, then they may be overweight. This is not heathy and if your cavy is dramatically heavier than 2.6 pounds, then they likely require more exercise, less food, or both.

Housing

Housing also plays a vital role in the health of your guinea pig. They should have as big as an enclosure as possible which is cleaned regularly, removing any soiled bedding or rotten food.

This will not only lead to a happier cavy, but also a healthier one, as a clean environment reduces the risk of diseases such as pneumonia.

Guinea pigs also need plenty of mental and physical stimulation.

This can be achieved by including various toys and tubes in the enclosure that allow guinea pigs to display natural behavior patterns.

Companionship

Being very social animals, guinea pigs enjoy each other’s company and are healthier when kept with a partner. If keeping boars and sows together, ensure they are neutered to prevent unwanted breeding.

Studies have shown that a lack of companionship can increase your guinea pigs stress levels, resulting in poorer health. This can result in less food intake and therefore slower growth.

This particular study even showed that a male guinea pig lost  20.9% of their initial body weight during long-term isolation.

As you can see, there can be a lot of variation in size between guinea pigs and it is very important to maximize on the areas mentioned above to ensure healthy growth.

If you are concerned that your guinea pig is not growing as quickly as it should or if they seem otherwise unwell, contact your local veterinarian immediately.

Finally

While guinea pigs are not the largest of pets that you could keep; that does not mean that they are necessarily considered to be small.

In fact, in the sub-category of ‘rodents’, they are the largest that you could look to own.

And beyond this, they do have specific requirements and needs for space. This is especially true of their housing.

They need sufficient room and floor space to roam and exercise, with separate places to nest, dig and burrow.

In other words, “size matters”, as the Humane Society succinctly puts it.

And you have to consider their needs in relation to how many you are looking to keep.

The more guinea pigs, the more space you will need to afford.

For instance, you will need to take the following recommendations into consideration when looking at cages:

Number of Guinea Pigs OwnedMinimum Cage SizePreferential Cage Size
17.5 Square Feet10 Square Feet
27.5 Square Feet10.5 Square Feet
310 Square Feet13 Square Feet
413 Square Feet16 Square Feet

Then there is where you place the enclosure, the kind of conditions you keep and how you care for and clean the cage.

But these are topics for another day.

Ultimately, just as with any pet, its important to consider their final size and what this could mean in regards to their ownership.

Can you realistically provide what they need? That is the question to ask.