Convert your cat’s age to human-equivalent years. The first year counts for fifteen, the second for nine, and every year after for four.

The conversion

Year 1        → 15 human years
Year 2        → 9 more, so 24
Every year after → 4 more

So a three-year-old cat is about 28, a ten-year-old is about 56, and a twenty-year-old is about 96.

Why “multiply by seven” is wrong

Seven times one is seven, and a one-year-old cat is not a seven-year-old child. It is fully grown, sexually mature, and capable of having kittens of its own.

Ageing is front-loaded. A kitten does in twelve months roughly what a human does in fifteen years, and the curve flattens sharply after that. Any single multiplier has to be wrong at one end or the other, and seven is wrong at both:

Cat’s age	Times seven	Actual equivalent
1	7	15
2	14	24
5	35	36
10	70	56
15	105	76

The two curves cross at about five, which is why the rule survives — it happens to be roughly right in the middle of a cat’s life and badly wrong at both ends.

Life stages

The AAFP and AAHA feline life stage guidelines divide a cat’s life into four, and these are the stages a vet will use:

Stage	Age	Roughly
Kitten	under 1 year	0–15
Young adult	1–6 years	15–40
Mature adult	7–10 years	44–56
Senior	11 years and over	60+

The practical significance is veterinary rather than sentimental: a mature cat is usually recommended annual check-ups and a senior cat twice-yearly ones, because the conditions that appear in later life — kidney disease, hyperthyroidism, arthritis, dental disease — progress quietly and are far more treatable when caught early.

Kittens, month by month

The first year is compressed enough that months matter:

Kitten’s age	Roughly
1 month	1 year
3 months	4 years
6 months	8 years
9 months	11 years
12 months	15 years

A six-month-old kitten is an adolescent, which is why it can already reproduce and why neutering is usually recommended before then.

Indoor and outdoor cats age differently

The table above describes ageing, not life expectancy, and those diverge sharply:

	Typical lifespan
Indoor cat	12–18 years
Indoor–outdoor cat	10–15 years
Outdoor cat	2–5 years in some studies

The difference is not biological ageing but accident, disease and predation. An indoor cat reaching twenty is unremarkable; the record is over thirty.

What actually shows a cat’s age

None of this is a measurement. A vet estimates an unknown cat’s age from teeth first — eruption in kittens, then wear and tartar — and from the eyes, coat condition and muscle tone. Blood work refines it. For a cat with no history, the estimate is a range, not a number.