Convert your dog’s age to human years using the veterinary table, with the size adjustment — and the 2020 epigenetic formula that disagrees with it.

The conversion

Year 1        → 15 human years
Year 2        → 9 more, so 24
Every year after → 4, 5 or 6 more, depending on size

Size only matters after the second year. Every dog reaches adulthood at roughly the same point; what differs is how fast it ages afterwards.

Adult weight	Per year after two
Small, under 10 kg	4
Medium, 10–25 kg	5
Large, over 25 kg	6

By size and age

Dog’s age	Small	Medium	Large
1	15	15	15
2	24	24	24
5	36	39	42
8	48	54	60
10	56	64	72
13	68	79	90
15	76	89	102

Big dogs age faster and die younger, which is unusual — across species, larger animals generally live longer. Within a single species it runs the other way, and why is still an open question. A Great Dane is old at seven; a chihuahua at seven is middle-aged.

Why “multiply by seven” survives

Because it is roughly right for a medium dog in the middle of its life, and wrong everywhere else:

Dog’s age	Times seven	Medium dog, actual
1	7	15
2	14	24
6	42	44
12	84	74

A one-year-old dog is not a seven-year-old child. It is fully grown and can reproduce.

The 2020 epigenetic clock disagrees

A 2020 study by Wang and colleagues compared DNA methylation — chemical marks that accumulate on DNA with age — in Labradors and humans, and produced a formula:

human years = 16 × ln(dog years) + 31

It gives very different answers at the young end:

Dog’s age	Table (medium)	Epigenetic
1	15	31
2	24	42
5	39	57
10	64	68
14	84	73

Both are shown here because the disagreement is real and informative. The table matches life stage — what the dog can do, how it behaves, what a vet screens for. The formula matches molecular ageing — how worn the genome looks. They converge around ten, which is when the two meanings of “equivalent” happen to coincide.

The formula was derived from a single breed and is undefined below a year, where the logarithm runs away and the study had no data.

Life stages

The AAHA canine life stage guidelines use four stages, and the boundaries move with size:

Stage	What matters
Puppy	growth, socialisation, vaccination
Young adult	fully grown, highest activity
Mature adult	middle age; weight and dental disease appear
Senior	screening frequency doubles

Senior begins at a different age for different dogs. A giant breed is senior at around six, a medium dog at eight to ten, a small dog at ten to twelve. The calculator above uses a single simplified boundary and does not model this; a vet will not.

Typical lifespans

Size	Years
Small, under 10 kg	12–16
Medium, 10–25 kg	10–14
Large, 25–40 kg	9–12
Giant, over 40 kg	7–10

Breed matters as much as size within these ranges, and mixed-breed dogs tend to outlive pedigree dogs of the same weight.

None of this is a measurement

It is a convention for translating between two lifespans that do not have the same shape. A vet estimating an unknown dog’s age uses teeth, eyes, coat and muscle tone, and gives a range.

Dog age in human years