Adult rats usually weigh 300–450 g (10.6–15.9 oz) (does) and 450–650 g (15.9–22.9 oz) (bucks) (Merck), and reach full size at about 5–6 months. For example, a typical 8-week-old buck weighs 218–354 g (7.7–12.5 oz), and a 150 g rat of unknown sex is probably 5–8 weeks old.
Estimates only. There is no veterinary growth standard for pet rats. These weights come mostly from lab Sprague-Dawley rats — the same species as pet rats — plus pet-rat figures from the AFRMA, NFRS and Merck. Pet rats vary, and body condition matters more than any number.
Fancy rat age size chart
This fancy rat age size chart gives typical weights by age for does (females) and bucks (males), from birth to adult. It's the same rat weight chart — rat size by age, in grams and ounces — that the calculator uses, and between rows the calculator draws a straight line. Use it the other way round, too: as a sizing rat age size chart, find the row your young rat's weight fits to read off a likely age.
| Age | Female (doe) | Male (buck) |
|---|---|---|
| Birth | 5–7 g (0.2 oz) | 5–7 g (0.2 oz) |
| 7 days | 11–14 g (0.4–0.5 oz) | 11–14 g (0.4–0.5 oz) |
| 2 weeks | 25–31 g (0.9–1.1 oz) | 25–31 g (0.9–1.1 oz) |
| 3 weeks | 34–54 g (1.2–1.9 oz) | 34–56 g (1.2–2 oz) |
| 4 weeks | 53–92 g (1.9–3.3 oz) | 61–92 g (2.1–3.3 oz) |
| 6 weeks | 116–195 g (4.1–6.9 oz) | 135–260 g (4.8–9.2 oz) |
| 8 weeks | 157–241 g (5.5–8.5 oz) | 218–354 g (7.7–12.5 oz) |
| 3 months | 213–261 g (7.5–9.2 oz) | 327–413 g (11.5–14.6 oz) |
| 4 months | 279–341 g (9.9–12 oz) | 368–492 g (13–17.3 oz) |
| 6 months | 300–449 g (10.6–15.8 oz) | 449–649 g (15.9–22.9 oz) |
| 1 year | 300–450 g (10.6–15.9 oz) | 450–650 g (15.9–22.9 oz) |
| Adult | 300–450 g (10.6–15.9 oz) (Merck) | 450–650 g (15.9–22.9 oz) (Merck) |
The two sexes weigh about the same for the first four weeks. From about six weeks bucks pull clearly ahead, and by adulthood they are roughly half as heavy again: Merck gives adult does 300–450 g (10.6–15.9 oz) and bucks 450–650 g (15.9–22.9 oz). The NFRS describes typical adults as "around 300 g" for does and "around 500 g" for bucks.
Where each row comes from
Ranges built from published averages are widened by 10% on each side, because individual rats vary around any average — that margin is our assumption, not a published figure. Birth and adult rows use published ranges as they are.
| Age | Basis |
|---|---|
| Birth | Lab rats: 5–7 g at birth (Ghasemi 2021) |
| 7 days | Sprague-Dawley averages (Ghasemi 2021) |
| 10 days | About 20 g by day 10 (Ghasemi 2021) |
| 2 weeks | About 30 g by day 15 (Ghasemi 2021) |
| 3 weeks | Lab growth charts (Envigo, Inotiv) and Ghasemi 2021 |
| 4 weeks | Lab growth charts (Envigo, Inotiv) and Ghasemi 2021 |
| 6 weeks | Lab charts + AFRMA pet-rat guide (6 oz / 8 oz) |
| 8 weeks | Lab growth charts and Ghasemi 2021 |
| 3 months | Envigo Sprague-Dawley chart (12 weeks) |
| 4 months | AFRMA: 4-month does about 11 oz; bucks interpolated |
| 6 months | Adult ranges (Merck; NFRS) |
Baby rat age chart: development week by week
For babies, development is a better clock than weight. This baby rat age chart lists the milestones in order, so you can match what you see to an age.
| Age | What you'll see | Source |
|---|---|---|
| Birth | Born pink, deaf and blind; 5–7 g | Merck; Ghasemi 2021 |
| 2–4 days | Ears open | NFRS |
| About 1 week | A coat shows; weight roughly doubles to about 12 g | NFRS; Ghasemi 2021 |
| 8–10 days | Incisors come through; plush coat by about 10 days | NFRS |
| About 14 days | Eyes open (14–17 days) | NFRS; Animal Diversity Web |
| 2½–3 weeks | Starts eating solid food | ICWDM |
| About 21 days | Weaned | Merck |
| 3–4 weeks | Independent of the mother | ICWDM |
| About 5 weeks | Can leave the mother | NFRS |
| About 6 weeks | Sexually mature — separate bucks and does | Sengupta 2013 |
| After 8 weeks | Adult, still growing | Sengupta 2013; Ghasemi 2021 |
| 5–6 months | Weight gain becomes negligible | Ghasemi 2021 |
When do baby rats open their eyes? At about 14 days (NFRS), or 14–17 days by the Animal Diversity Web's account. When can baby rats leave their mother? They are weaned at about 21 days (Merck) but the NFRS separates mother and young at about five weeks, which gives them time to learn from her. Separate the sexes by then, because rats are sexually mature at about six weeks.
Baby rat weight chart
Lab rats weigh 5–7 g at birth, about 12 g at one week, about 20 g by day 10, about 30 g by day 15 and around 38–51 g at weaning (Ghasemi 2021; Envigo/Inotiv growth charts). By four weeks most weigh about 60–85 g, and by six weeks about 130–235 g. Pet litters can be a little lighter or heavier depending on litter size and the mother's milk.
How big do pet rats get?
How big do rats get as pets? Merck's "Rats at a Glance" puts the body at 9–11 inches (23–28 cm) long, with a tail of another 7–9 inches (18–23 cm). That makes a full grown rat about 16–20 inches nose to tail tip. Adult rat weight is 350–450 g (12–16 oz) for does and 450–650 g (16–23 oz) for bucks (Merck).
- How big do fancy rats get? The same as any pet rat: fancy varieties differ in coat and ears, not in basic size.
- Male vs female rat size: bucks are bigger, heavier and broader, often with a coarser coat; does are slimmer and more active.
- Full grown rat size: reached at about 5–6 months, when weight gain becomes negligible in lab rats (Ghasemi 2021). Lab studies show slow gains can continue for many months in males, so some bucks fill out a little more.
Rat growth curve
The chart below shows the typical weight and range for males and females from birth to 6 months, using the table above. The steep part of the curve, between about 3 and 8 weeks, is when weight tells you the most about age.
Can you tell a rat's age from its size?
Yes, but only while it's young. Weight is a good clue in the first two months, when rats gain tens of grams a week, and a rough clue to about 5–6 months. After that, weight gain is negligible (Ghasemi 2021), so how to tell a rat's age by size has a simple answer for adults: you can't. A 400 g buck could be six months or two years old.
For example, a 60 g rat of unknown sex is probably 3–4 weeks old, and a 150 g rat 5–8 weeks. Adding eyes and coat narrows the range for babies. To turn an age into human years and a life stage, use our rat age calculator.
- Litter size, the mother's milk, diet and illness all shift weight, so a small rat can be older than it looks.
- Most rows come from lab Sprague-Dawley rats. Pet rats come from many different lines, so yours may run lighter or heavier at the same age.
How much should my rat weigh?
How much should my rat weigh as an adult? Usually 350–450 g for a doe and 450–650 g for a buck (Merck), though healthy pets outside those ranges are common. For a young rat, use the "Age → expected weight" mode above with your rat's age and weight to see whether it's below, within or above the typical range.
Is my rat overweight?
The scale only tells half the story. Feel your rat: you should be able to feel the spine and hip bones easily under a thin layer, but they shouldn't feel sharp. A rat whose bones you can't find, with a round belly and fat around the base of the tail, is probably overweight; a rat with sharp, obvious bones is too thin. PetMD notes that weight loss is the most common first sign of illness in pet rats, so weigh weekly and see an exotic-animal vet about any unexplained loss — or if you're not sure whether your rat's weight is right.
- Weigh at the same time of day on a kitchen scale in grams.
- Write the weights down: the trend over weeks matters more than one number.
- Cut back on treats before cutting the main diet, and keep a complete rat food as the base (RSPCA).
How this rat size calculator works
Weight → estimated age: the calculator finds every age at which your weight falls inside the typical band for the sex you chose (both sexes if you're not sure). Optional signs then narrow the range: closed eyes mean about 0–17 days and open eyes 14 days or more; "not yet furred" means about 0–10 days. If the signs disagree with the weight, it trusts the signs, because development is more reliable than weight in babies. Once a rat reaches adult weight — 300 g for does or 450 g for bucks — or a likely age of 5 months or more, it reports "adult-sized" instead of an age.
Age → expected weight: it reads the band for that age and sex from the table, and if you add a weight it reports below, within or above the range with a range bar. It uses the same weight-range engine as our cat growth charts. Weights outside 4–1,000 g show an error, as no rat weighs that.
Frequently asked questions
How big do rats get?
Pet rats grow to about 9–11 inches (23–28 cm) long plus a 7–9 inch tail (Merck). Adult does usually weigh 300–450 g (10.6–15.9 oz) and bucks 450–650 g (15.9–22.9 oz).
How much should a 6 week old rat weigh?
About 116–195 g (4.1–6.9 oz) for a doe and 135–260 g (4.8–9.2 oz) for a buck, based on lab growth charts and the AFRMA pet-rat guide.
How much should an 8 week old rat weigh?
About 157–241 g (5.5–8.5 oz) for a doe and 218–354 g (7.7–12.5 oz) for a buck.
When do baby rats open their eyes?
At about 14 days (NFRS); the Animal Diversity Web gives 14–17 days.
When can baby rats leave their mother?
They are weaned at about 21 days (Merck), but the NFRS keeps them with their mother until about 5 weeks.
When is a rat full grown?
At about 5–6 months, when weight gain becomes negligible in lab rats (Ghasemi 2021). Some bucks fill out slowly for longer.
Can you tell a rat's age by its size?
Only while it's young. Weight is a good clue for the first two months and a rough one to about 5–6 months; after that, adult rats of very different ages weigh the same.
Are male rats bigger than female rats?
Yes. Merck gives adult bucks 450–650 g and does 350–450 g; the NFRS says around 500 g and 300 g.
Is my rat overweight?
Check body condition: you should feel the spine and hips easily, but not sharply. If you can't find the bones, your rat may be overweight; ask an exotic vet for a plan.
This calculator gives an estimate for education and fun. Every pet ages differently — for health decisions, talk to your veterinarian.