How we calculate pet age
PetAgeNow turns a pet's age and selected dog size into a general human-age estimate. Here is how our tables work and what the result can tell you.
What the estimate means
Human years are a way to compare ages. Our calculator uses its own reference tables; it has not been clinically validated as a measure of biological age. A result does not predict a pet's health, behavior, or remaining lifespan. Breed, size, genetics, and health can affect how individual pets age.
From years and months to a result
We add years plus months divided by 12, then look up that age in the cat table or your selected dog-size table. All five tables assign 0 human years at birth, 10 at 6 months, 15 at 1 year, and 24 at 2 years. These are model assumptions, not measured developmental milestones.
Between and beyond the table entries
Between listed ages, we interpolate in a straight line and round the result to the nearest whole human year. For example, 1 year 6 months falls halfway between 15 and 24: 19.5, displayed as 20. Before 6 months we scale from zero. Dog tables end at 16 years and the cat table at 20; beyond those ages we extend the final yearly increase (small 4, medium 4, large 5, giant 7, cat 4). Those extended estimates are especially uncertain.
How size and breed are used
After age 2, dogs use different reference values for each size. The calculator uses the size you select; it does not classify a weight automatically. The weight labels are broad guides with a shared boundary at 55 lb. Breed pages only preselect a size that you can change. They do not use separate breed-specific aging equations.
Cat estimates and life-stage labels
In our cat table, each year after age 2 adds 4 human years. The calculator's life-stage labels are broad age-based descriptions. They do not assess an individual pet and do not reproduce current AAHA life-stage definitions. For example, AAHA/AAFP groups cats over 10 as senior, while this site also uses a Super Senior label after 15. Ask your veterinarian about your pet's actual care needs.
| Dog age | Small | Medium | Large | Giant |
|---|---|---|---|---|
| 1 year | 15 | 15 | 15 | 15 |
| 2 years | 24 | 24 | 24 | 24 |
| 5 years | 36 | 37 | 39 | 45 |
| 8 years | 48 | 51 | 54 | 64 |
| 10 years | 56 | 60 | 64 | 79 |
| 13 years | 68 | 74 | 79 | 100 |
| 15 years | 76 | 83 | 88 | 114 |
Small: Under 22 lb; Medium: 22-55 lb; Large: 55-99 lb; Giant: Over 99 lb.
Explore the full dog age chart| Cat age | Human-age estimate |
|---|---|
| 1 year | 15 |
| 2 years | 24 |
| 5 years | 36 |
| 8 years | 48 |
| 10 years | 56 |
| 15 years | 76 |
| 20 years | 96 |
Sources and limitations
These resources explain veterinary life stages and individual variation. They provide context, not the exact numerical tables above, and do not endorse or validate PetAgeNow's calculator.