Kitten weight by week: what the growth data shows

From birth to 8 weeks

The Merck Veterinary Manual gives a normal birth weight of 90 to 100 g (3.2 to 3.5 oz) for kittens. A 2024 study of 5,504 kittens born in 193 French catteries found a median birth weight of 101 g, with the middle half between 89 and 115 g and a full range from 36 to 182 g. In that study, kittens that lost weight, or failed to gain, between birth and day 2 had a higher risk of dying in their first two months.

After the first days, Merck says growth is very fast for the first 3 to 4 months, with kittens gaining 50 to 100 g (1.8 to 3.5 oz) a week. That gives a simple check before the growth charts begin: weigh twice, a week apart, on the same scale. For example, a kitten that goes from 350 g at 4 weeks to 410 g at 5 weeks gained 60 g, inside Merck’s range. One that went from 350 g to 380 g gained 30 g, below it — call your vet. A kitten under 8 weeks that loses weight or stops gaining should be seen promptly.

Typical weights from 8 to 78 weeks

The 2022 standards start at 8 weeks, the same age at which WALTHAM says its kitten charts are validated. Here are the medians and the middle half (25th to 75th centile) from our summary of the study’s modelling data:

So, is an 8-week-old big enough? Half of the healthy kittens in the study were between about 1.6 and 2.5 lb at that age, and nearly all — the 2nd to the 98th centile — were between 0.53 and 1.62 kg (1.2 to 3.6 lb) for females and 0.45 and 1.73 kg (1.0 to 3.8 lb) for males. A 900-g (2.0-lb) kitten at 8 weeks sits close to the middle for either sex.

These standards cover domestic shorthair kittens only; the study’s authors filtered out most weights above 6.5 kg as unlikely for a domestic shorthair cat in normal body condition. WALTHAM describes its kitten charts as suited to cats with an adult weight of 2 to 7 kg.

Females and males grow apart

At 8 weeks the sexes weigh about the same. After that, males gain faster. Between 8 and 16 weeks the median weight rose by about 117 g a week for females and about 149 g a week for males. By 16 weeks the median male was 12% heavier than the median female, and by 1 year 42% heavier.

Notice that both weekly gains are above Merck’s 50 to 100 g a week. The two sources describe different things — Merck gives a general range for young kittens, the data show what healthy U.S. pet kittens actually gained — so a kitten gaining more than 100 g a week at this age is in line with the healthy kittens in the study. Gaining less than 50 g a week, or losing weight, is the signal to call your vet.

Neutering also changes the curve. A 2023 study from the same group found that kittens’ median growth path bent upward after neutering, more in females than in males, so weight is worth watching closely in the months after the operation.

When do kittens stop growing?

Sources differ, and the honest answer is a range: most kittens finish growing at about 7 to 12 months.

• Merck says the growth rate begins to plateau at 150 to 160 days and that growth is usually complete within 200 to 220 days — about 6½ to 7 months.
• A UK research colony of intact domestic shorthair cats, part of the same open dataset, gives a slower finish. Measured against each cat’s own weight at 1½ years or older, the typical kitten had reached 68% at 26 weeks, 84% at 39 weeks, 91% at 52 weeks and 96% at 65 weeks. It passed 90% at about 48 weeks (11 months) and 95% at about 63 weeks (14½ months).

Merck adds that kittens are generally moved to an adult diet sometime between 8 and 12 months; your vet can help you choose when.

Is one pound a month right?

A popular rule says a kitten weighs about a pound for each month of age, until about 6 months. Against the U.S. data it is a fair rough guide for females and runs light for males:

For females the rule stays within about 11% of the median through 6 months; for males it runs 7% to 20% low. Like any one-number rule, it says nothing about the spread, so the centile bands are the better comparison.

Centile lines, and when to call your vet

A centile line marks the weight below which a given share of healthy kittens of that age and sex fell. Healthy kittens do cross lines: in the study’s comparison group of research-colony kittens, 28% crossed at least two lines upward and 23% at least two lines downward. Among 122 kittens that were overweight, 52% had crossed at least two lines upward. The authors present the standards as an aid for veterinary teams deciding whether a kitten’s nutrition and care need to change.

Call your vet if your kitten:

• loses weight, or stops gaining, at any age before it is grown,
• gains less than 50 g a week in its first 3 to 4 months,
• sits below the lowest line for its age and sex, or keeps falling across lines, or
• seems unwell, whatever the scale says.

This is general information, not veterinary advice — talk to your vet about your own kitten.

Next step

Weigh your kitten every 2 to 4 weeks — the schedule WALTHAM suggests up to 6 months — and enter each weight in the kitten weight chart. It shows where your kitten sits among the centile lines for its sex and age, flags a weekly gain under 50 g before 8 weeks, and keeps the whole path in one view. When it is time to plan meals, the guide on how much to feed a cat explains calorie needs.