About the Kitten Age Calculator
This kitten age calculator estimates how old a kitten is when you do not know its birthday — the usual situation with a found litter, a foster or a rescue. Enter the kitten’s weight on a kitchen scale, or pick the developmental milestone it has reached (eyes open, ears up, teeth showing, weaned), and you get an estimated age in weeks, the likely range, its stage and what it needs to eat right now.
Healthy kittens follow a remarkably consistent pattern: roughly 100 g (3.5 oz) at birth, gaining about 100 g a week for the first two months, then close to a pound a month until five or six months. That makes weight the best single age clue for young kittens, which is why shelters and foster programmes use weight-by-age charts.
Treat the result as an estimate. Breed, litter size and illness all affect growth — an underweight or sick kitten will look younger than it is, so combine weight with milestones and have a vet confirm age before vaccinations. A kitten that is not gaining weight daily in its first weeks needs veterinary attention.
With the default inputs, the estimated age is 4 weeks. Change any value above to recalculate instantly.
How to use the kitten age calculator
- 1Weigh the kitten on a kitchen scale in grams or ounces (or choose the milestone method).
- 2Enter the weight and unit, or pick the most advanced milestone the kitten shows.
- 3Read the estimated age in weeks and the likely range.
- 4Follow the feeding guidance for that age and compare with the weight chart.
- 5Ask a vet to confirm the age before scheduling vaccinations.
Formula and method
Kittens weigh about 100 g at birth and gain roughly 100 g (10–15 g a day) each week until about 8 weeks, so up to about 900 g the calculator estimates age as (weight − 100) ÷ 100 weeks. After that, kittens gain close to a pound a month until about six months, so the calculator draws a straight line from 900 g at 8 weeks to 6 lb (2,722 g) at 26 weeks — about 101 g a week — and reads the age off that line, which keeps the curve smooth at the 8-week join. The likely range adds about ±15% plus half a week for normal variation.
The milestone method uses typical ages for each developmental sign — umbilical cord drops off at 3–5 days, eyes open at about 7–14 days, ears stand up and kittens walk at around 3 weeks, baby teeth appear from 3–4 weeks, weaning finishes around 8 weeks, and adult teeth replace baby teeth from about 3 to 6 months — and reports the midpoint.
- grams
- Kitten’s current weight in grams
- 100 g
- Typical birth weight and weekly gain
- 101 g
- Typical weekly gain from 8 to 26 weeks (900 g to 6 lb)
Worked examples
500 g kitten
A 500 g kitten has gained about 400 g since birth (100 g). At roughly 100 g a week that is about 4 weeks old — the age to start a wet-food gruel and a litter tray.
3 lb kitten
At 3 lb (about 1,361 g) the kitten is past the 100 g-a-week phase. It is 461 g above the 900 g typical at 8 weeks, and at about 101 g a week that adds roughly 4.5 weeks, so it is around 12½ weeks (just under 3 months) old.
Eyes open, still crawling
Eyes open at about 1.5 weeks and kittens start walking around 3 weeks, so a kitten with open eyes that is still crawling is about 2¼ weeks (16 days) old and still needs bottle or mother feeding every few hours.
Frequently asked questions
How can I tell how old a kitten is?+
Weight is the best single clue for young kittens: about 100 g at birth plus roughly 100 g per week to 8 weeks, then about a pound per month. Confirm with milestones — eyes open at 1–2 weeks, ears up and walking around 3 weeks, baby teeth from 3–4 weeks.
How much should a 4-week-old kitten weigh?+
A healthy 4-week-old kitten usually weighs about 450–550 g (16–19 oz). A kitten well below that may be younger, or may be underweight and need a vet check.
When do kittens open their eyes?+
Kittens are born with their eyes closed and usually open them between 7 and 14 days of age. All kittens have blue eyes at first; the adult colour starts to appear around 6–7 weeks.
When can kittens leave their mother?+
Kittens should stay with their mother and litter until at least 8 weeks, and many shelters and breeders wait until 10–12 weeks so kittens are fully weaned and better socialised.
How much weight should a kitten gain each day?+
Healthy kittens gain about 10–15 g a day (roughly 100 g a week) in the first two months. Weigh young or fostered kittens daily at the same time; no gain or any loss for a day or two is a reason to call the vet.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.