There’s a lot of fat shaming in the rat community and I have personally experienced histrionic aggression aimed at my parenting skills, because a couple of my rats have been obese.

Rats who’d had blood tests by which endocrine disorders were confirmed, but of course judgemental and/or aggressive people wouldn’t think to presume a rat can be fat, AND happy and well cared for – or that it wasn’t due to my care skills.

Obesity and overweight in rats is not a straightforward matter.  A lot more understanding and a lot less jumping to conclusions would make the rat community a nice place to share.

Which brings me to a request to write an article on the healthy weight of growing rats.

The average ranges of healthy adult rat weights are known, but what is a healthy weight range for a three or five-week-old rat, or a three-month-old one, for instance?

**This is not an advice site, but what I can and will continue to do is report science-based facts from which readers can draw their own conclusions.



Rats are tested on in laboratories to gather data on human-related issues, not to gather data on rats.

Assembling the science regarding how rats are affected by their environments etc, is largely a matter of cobbling together studies that don’t relate to the rat issue, and extracting what is pertinent to the rat, not the pharmaceutical, procedure or human etc.

For instance, as I trawled through peer-reviewed journal databases for information on rat weights, I had to open dozens of articles in the hopes one of them mentioned the weight of young rats; studies about medical issues I can’t begin to understand and that  aren’t concerned with the actual rat or its wellbeing.

In short – it’s bloody time consuming and mostly turns up nothing applicable to the rats themselves.

Studies on rats are conducted on lab rats whose environments are highly controlled unlike the environments of their cousins – our pet fancy rats.

This means data should be taken with a grain of salt, as unscrupulous breeders have been doing their own thing for decades including breeding specifically for size, and far too often, without regard for health.

Therefore, consider whether it’s appropriate and right to strictly believe a lab rat’s optimal weight is a definitive guide to fancy pet rat weights.

I’ve had two rats that were from a line that had been deliberately bred for size, and they didn’t stop growing in length until they were very old.

Amis was one of two rats I’d had (years apart), that came from an unscrupulous breeder who bred for size without regard for health.
They were both mammoth rats, but Amis’ weight was increased due to his health issues.
He lived to 2 years 8 months after being in palliative treatment for 10 months due to his myriad of health problems.
One of the best cared for and well medically treated rats in history.

They were naturally big boys – well beyond the “normal” lab rat optimal maximum weight, and length, and their health wasn’t at all affected because of that (vet confirmed!).



In a study on feeding behaviour and body weight development where rats were given gastric bypasses (Furnes et al. 2009), the following results have been extracted:

  1. Rat weight doubles in the first five days after birth.
  2. After weaning, a rat’s average weight is 45g.
  3. By 250-300 days, most rats will have reached their maximum size.
  4. Some rats will continue to grow throughout their lives, like the Sprague-Dawley (lab) rat.
  5. Most of the weight an adult rat naturally gains fat being necessarily distributed.

Johns Hopkins University’s Animal Care and Use Committee reported:

  1. Rat birth weight is 5-6g.
  2. Normative adult weight for a male is 300-500g.
  3. Normative adult weight for a female is 250-300g.

The Western Australia Government Animal Resources Centre reported on the average weight range of different rat breeds in their care between the ages of 21 days and 84 days:

  • Male rats at 21 days old across all breeds ranged from 24g to 96g.
  • Female rats at 21 days old across all breeds range from 22g to 75g.
  • 42 day old males – 89g to 337g.
  • 42 day old females – 61g to 196g.
  • 56 day old males – 155g to 375g.
  • 56 day old females – 105g to 224g.
  • 84 day old males – 182g – 492g.
  • 84 day old females – 147g – 297g.

As you can see the ranges are big!  These are also only averages so the differences in weight would be far more interesting a story than these averages are telling.

I crunched the numbers (uuuuggghhh) and came up with the average weight gain for each breed from 21 days old to 84 days old, and then despite failing maths in high school, miraculously worked out the average weight gain due to natural growth for all breeds.

The answer is…males put on an average of 267g, and females gain an average of 162g.

I was unable to find information about rat weights between the ages of three and ten months, but we can still draw some common sense conclusions about what this information does tell us.



84 days is less than three months old and as reported above, weight gain doesn’t usually slow down and stop until around 10 months old.

These studies and sources are by no means an exhaustive list of evidences, but they give us a good indication of just how much weights can vary, which is natural, healthy and normal.

If an 84 day old rat can already be just shy of 500g with seven months still to grow, it’s safe to presume that it wouldn’t be natural and normal for that rat to be 500g fully grown, as per John Hopkin’s normative maximum.

Wistar rats are the heaviest rats according to the above data, and our fancy rats are closely related to them.

They are outbred albino lab rats…remind you of anything?

lab rat fancyrat rats pet rats pocket pets

Yep!  They look like your fancy rats don’t they?!  Perhaps heavier than the average is a genetic destiny, but of course that’s for you to decide for yourself.

Based on what I could find regarding weight, the answer to “what’s a normal and healthy weight for a young rat?” is one that you as a thinking human with eyeballs can probably figure out.

Healthy weights will vary.

By looking at literally thousands of rats online for the past seven years, I have concluded that I can tell when a rat’s weight is looking “abnormal”, and I haven’t been able to definitively correlate overweight to ill health or early death.

I also know from personal experience that just because a rat’s weight might look abnormal, doesn’t mean they aren’t overall very healthy, very happy, and can’t live well past the average age.

Basil’s fat distribution triggered me to take him to the vet, where a blood test confirmed an endocrine issue.
He lived to three years old, happy as Larry, and died due to a recurring malignant mammary tumour.

Does abnormal weight definitively cause or signify ill-health?  In my personal experience, it does not.

Fat isn’t necessarily bad and tiny isn’t necessarily best.  Is your rat healthy?  That’s the most important question to ask yourself I say.

The Rat Guide’s health check checklist is marvellous and a good place to focus other than solely on weight, would you agree?

What is your personal experience either with your own rats or knowing other people’s rats, regarding weight?

Have you found anecdotally that there are hundreds of overweight rats out there that are happy, healthy and living to a ripe old age?  Or being taken by something completely unrelated like a pituitary tumour?

It’s important we share this information in the comments, go ahead, it’s a safe space!


Furnes, M W., Zhao, C M., Stenstrom, B., Arum, C J., Tommeras, K., Kulseng, B., Chen, D., 2009, Feeding bahavior and body weight development: lessons from rats subjected to gastric bypass surgery or high fat diet, ‘Journal of Physiology and Pharmacology’, vol 60, pp. 25-31.



Melanie is a freelance copywriter, blogger and rat fan!You can hire her to write words that suit your purposes and achieve your goals, just shoot her an email through the Contacts page.
Melanie shares her flat with two female ginger cats and occasionally some ants.

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  1. Thank you for the info I too have read so many articles just to get a smidge of info. I currently have two girls Tempest & Cubby who I have had since birth, they are double the size of my boys they both weigh about 700g. Tempy doesn’t look as big as Cubby, Cubby has a smaller frame and she has lipoma along the left side of her body the vet told me there was no need to operate. Cubby also has two pea size mammary tumors one on the right side of her back and another between her front legs. Which I discussed with her vet but as they haven’t grown in size I have decided not to operate. Cubby was also the runt of the litter her mother Storm was tiny probably less than 3 months old when she gave birth to them (irresponsible pet shop) she gave birth to a litter of 8. Cubby has cried her whole life and is not overly fond of being held. My vet and I believe after trialling her on meloxicom that she most likely has some difformity from being squeezed in such a tiny space which has contributed to her gaining weight. Well that’s my story so far the girls are currently 25 months old and have outlived 5 of their siblings and are the oldest rats I’ve owned so far… Biggie (their brother) would most likely still be alive today if it weren’t for taking a vets advice against my better judgement but that’s another story. Thank you for reading.

    1. Hi Jarmarie thank you for telling your story. I think if there’s anything we can take from your story it’s what I’ve long suspected – we just really know very little about rats! They’re the most tested on animal on the planet, yet so much of their diagnosis and treatment is guess work. I miss ratty whinging, I had several squeakers and I’d love to have the air in my house filled with them again x

    1. I’m glad it was, thanks Aimee. If there’s anything I want people to take away from this blog it’s that just like humans, rat genetics can be anything! Just because one person has a health/fat/skinny/prone to tumours/allergic etc rat, doesn’t mean all rats are the same or will behave or be affected the same way as one.

      Being a newbie is a HUGE learning curve, always feel free to contribute suggestions and ideas AND pitch a blog post or two! I’d love to post a story about your experiences having rats for the first time x

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