How do you predict a kitten's adult weight from its current weight and age? The short answer is that you cannot predict it exactly — but you can estimate it, using the rough growth shares that vets use as a guide. This guide explains the two common methods: the growth-share method, which works from the kitten's weight as a share of its expected adult weight, and the doubling method, which works from the kitten's weight at a particular age. Both are rough estimates, and both are best used with the breed's expected adult weight, not instead of it. The kitten weight predictor does the same maths for your kitten's exact weight and age.
The short version is: take the kitten's current weight, divide it by the rough share for its age, and you get an estimate of its adult weight. At 8 weeks the share is about a fifth to a quarter, at 3 months about a quarter to a third, at 4 months about a third to two-fifths, at 6 months about half to two-thirds, at 9 months about three-quarters to four-fifths, and at 12 months about four-fifths or more. A kitten that is 5 kg at 6 months, when the share is about half to two-thirds, will be roughly 8 to 10 kg as an adult. That is the method in one sentence — and the rest of this guide explains when it works, when it does not, and how to use it with the breed's expected adult weight.
The growth-share method
The growth-share method works from the idea that a kitten grows along a rough curve, and that at each age it has reached a rough share of its expected adult weight. The shares vets commonly cite as a very rough guide are: about a fifth to a quarter at 8 weeks, about a quarter to a third at 3 months, about a third to two-fifths at 4 months, about half to two-thirds at 6 months, about three-quarters to four-fifths at 9 months, and about four-fifths or more at 12 months, when small breeds are near-adult and large breeds are still growing. To use it, take the kitten's current weight and divide it by the share for its age: a kitten that is 6 kg at 4 months, when the share is about a third to two-fifths, will be roughly 15 to 18 kg as an adult.
The method works best when you know the breed's expected adult weight and can check the estimate against it. If the growth-share estimate says the kitten will be 15 to 18 kg as an adult, and the breed's typical adult weight is 10 to 25 kg, the estimate is consistent with the breed — and you can use it to plan food and exercise. If the estimate says the kitten will be 40 kg as an adult, and the breed's typical adult weight is 10 to 25 kg, something is off: either the kitten is overfed, or it is a crossbreed with a larger parent, or the share for its age is wrong for its size. In that case, ask your vet to check the kitten's growth, rather than changing food on your own. The kitten weight chart by age guide gives the full shares by age, and the average cat weight by breed size guide gives the expected adult weights by size.
The doubling method
The doubling method is simpler: for medium-sized cats, a kitten's weight at about 14 weeks, doubled, is a rough estimate of its adult weight. A puppy that is 7 kg at 14 weeks will be roughly 14 kg as an adult, and a puppy that is 12 kg at 14 weeks will be roughly 24 kg as an adult. The method works best for medium breeds, and less well for very small or very large breeds: a toy breed grows faster relative to its adult weight, so doubling its 14-week weight overestimates its adult weight, and a giant breed grows for longer, so doubling its 14-week weight underestimates it.
For that reason, the doubling method is best used as a rough check, not as a prediction. If the doubling method and the growth-share method agree, and both agree with the breed's expected adult weight, you can be reasonably confident about the estimate. If they disagree, trust the breed's expected adult weight more than either method, and ask your vet to check the kitten's growth. The kitten weight predictor uses the kitten's current weight and age to estimate adult weight, and the when is a cat fully grown guide gives the growth ages by size and breed.
Worked examples by size
Take a small-breed kitten that will be about 7 kg as an adult. At 8 weeks it is roughly 1.5 to 1.75 kg, at 3 months roughly 1.75 to 2.3 kg, at 4 months roughly 2.3 to 2.8 kg, at 6 months roughly 3.5 to 4.7 kg, at 9 months roughly 5.25 to 5.6 kg, and at 12 months roughly 5.6 kg or more, when it is near-adult. Take a medium-breed kitten that will be about 17 kg as an adult: at 8 weeks roughly 3.4 to 4.25 kg, at 6 months roughly 8.5 to 11.3 kg, and at 12 months roughly 13.6 kg or more. Take a large-breed kitten that will be about 32 kg as an adult: at 8 weeks roughly 6.4 to 8 kg, at 6 months roughly 16 to 21 kg, and at 12 months roughly 25.6 kg or more, when it is still growing.
Those examples show why the method is rough: the ranges are wide, and a kitten at the top of its range at one age and the bottom at another is still normal. What matters is the line over weeks: a smooth, steady rise, not a flat line and not a sudden jump. A kitten that is not gaining weight at all over two weeks, or that is losing weight, needs a vet check. A kitten that is gaining weight in sudden jumps may be overfed, and a kitten that is gaining weight very slowly may be underfed or unwell. The how much food should a kitten eat per day by weight guide gives the calorie tables by weight, and the how often should you feed a kitten guide gives the feeding schedule.
When the estimate is wrong
The estimate is wrong most often for crossbreeds and mixes, because a crossbreed can take after either parent, or neither, and its adult weight can be far from either parent's weight. A crossbreed puppy from a small mother and a large father can be anywhere between the two, and no formula can tell you where. The estimate is also wrong for a kitten that is overfed or underfed: a kitten that is overfed will be heavier than the share for its age, and the formula will overestimate its adult weight, while a kitten that is underfed will be lighter, and the formula will underestimate it. That is why the breed's expected adult weight is the anchor, not the formula.
The estimate is also less reliable at very young ages: a kitten at 6 weeks has not yet settled into its growth curve, and a single weighing at that age can be misleading. Wait until 8 weeks or later for the first estimate, and re-estimate every two weeks as the kitten grows — the estimate gets better as the kitten gets older, because the shares get narrower and the kitten's own growth line becomes clearer. If you are worried about your kitten's growth, ask your vet to check it, and bring the weight record: a list of weights with dates is far more useful than a single number. The kitten too skinny — what to do guide covers a kitten that is too thin, and the how much should a kitten eat guide gives the calorie tables.
Using the estimate for feeding
Once you have an estimate of the kitten's adult weight, use it to plan food: a kitten that will be large needs more food than a kitten that will be small, and a kitten that will be large needs a large-breed puppy food, designed for steady rather than rapid growth. The amount must be re-checked every two weeks as the kitten grows, because the amount that was right a month ago is already wrong. Weigh the kitten every two weeks, find its new weight in the calorie table, and adjust the amount — that is the whole method, and it is worth doing, because a kitten fed last month's amount is already being fed wrong.
Do not use the estimate to restrict food: a kitten that is growing needs enough food to grow, and a kitten that is underfed will grow more slowly than it should. The estimate is a planning tool, not a diet plan. If the kitten is heavier than the estimate suggests, it may be overfed, and the amount should be cut a little — but if it is lighter, it may be underfed or unwell, and the vet is the right next step, not more food on your own. The how much food should a kitten eat per day by weight guide gives the calorie tables by weight, and the cat food calculator works out the figure for any weight.
For related reading, see the kitten weight predictor, the kitten weight chart by age guide, the how much food should a kitten eat guide, the average cat weight by breed size guide, the when is a cat fully grown guide, the kitten too skinny — what to do guide.
Frequently asked questions
How do I predict my kitten's adult weight?
Take the kitten's current weight and divide it by the rough share for its age: about a fifth to a quarter at 8 weeks, about a quarter to a third at 3 months, about a third to two-fifths at 4 months, about half to two-thirds at 6 months, and about four-fifths or more at 12 months. A kitten that is 5 kg at 6 months will be roughly 8 to 10 kg as an adult. Check the estimate against the breed's expected adult weight.
What is the doubling method for kitten weight?
For medium-sized cats, a kitten's weight at about 14 weeks, doubled, is a rough estimate of its adult weight. It works best for medium breeds and less well for very small or very large breeds. Use it as a rough check, and trust the breed's expected adult weight more than the formula.
How accurate is a kitten weight prediction?
Not very — it is a rough estimate, not a prediction. The ranges are wide, and a kitten at the top of its range at one age and the bottom at another is still normal. The estimate gets better as the kitten gets older, and it is least reliable for crossbreeds, for very young kittens, and for kittens that are overfed or underfed.
How often should I weigh my kitten?
Every two weeks, at the same time of day, on the same scales, and write the number down. A kitten grows fast, and the amount of food changes quickly, so a kitten weighed once a month is being weighed too rarely. Re-estimate the adult weight every two weeks as the kitten grows.
My kitten is heavier than the estimate — is it overweight?
Not necessarily. A kitten that is a little above the share for its age is usually fine, as long as it is gaining weight steadily and is bright, eating, and active. Feel the ribs: you should feel them easily under a thin layer of fat. If the kitten is much heavier than the share, ask your vet to check its growth rather than changing food on your own.
Can I use the estimate to plan feeding?
Yes — use it to choose the right food and the right amount, and re-check every two weeks as the kitten grows. But do not use it to restrict food: a kitten that is growing needs enough food to grow, and a kitten that is underfed will grow more slowly than it should.
Results on this page are estimates for general information only and are not veterinary advice. If your pet is ill, underweight, overweight, pregnant, or has a health condition, consult your veterinarian before changing diet or care.