Pet Age Calculator
Convert a pet age into the equivalent human age and see which life stage it is in.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
This is a rough species-and-size heuristic, not a clinical age equivalent or lifespan prediction. Breed, health, lifestyle and individual development change markedly how animals age; discuss life-stage care with a veterinarian.
Worked example
calculated with the default values below
Pet Dog · Age 5 years · Size (dogs only) Medium (25-60 lb) Equivalent human age 41.5 years · Life stage Adult — prime of life · Through its expected lifespan 43.5 % · Typical lifespan 12 years
General background references
Disclaimer
These calculators offer general guidance and do not replace veterinary advice. Individual animals vary with breed, weight, health status and medication. Suspected poisoning from chocolate, medication, plants or chemicals is an emergency — contact your veterinarian or a pet poison control service immediately rather than relying on a calculator.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
E = {14, s=1 ∧ z=1; 11.5, s=1 ∧ z=2; 9, s=1 ∧ z=3; 15, s=2; 9, s=3; 2.5, s=4; 28, s=5; 50, s=6}; H = {12.5y, s ∈ {1,2} ∧ y ≤ 2; 25 + (y−2)·{5.5, E > 10; 7.5, E ≤ 10}, s=1 ∧ y > 2; 25 + (y−2)·4.5, s=2 ∧ y > 2; 20, s=3 ∧ y ≤ 1; 20 + (y−1)·9, s=3 ∧ y > 1; min(90, 34y), s=4; 6.5, s=5 ∧ y ≤ 1; 6.5 + (y−1)·2.6, s=5 ∧ y > 1; 12, s=6 ∧ y ≤ 1; 12 + (y−1)·1.5, s=6 ∧ y > 1}; L% = y ÷ E × 100
- y
- Pet age in years
- s
- Species code (see note)
- z
- Dog size code (see note)
- E
- Average life expectancy (years)
- H
- Equivalent human age (years)
- L%
- Percentage of lifespan lived
s: 1 dog, 2 cat, 3 rabbit, 4 hamster, 5 horse, 6 bird. z (dog only): 1 small, 2 medium, 3 large. Life-stage thresholds at 15/50/75%.
Frequently asked questions
Why does a large dog age faster after year two than a small one?
The formula switches at age two: the first two years count as 25 human years total, then each later year adds 5.5 for dogs with expectancy over 10 but 7.5 for shorter-lived large breeds. A large dog (expectancy 9) at 5 therefore reads 25 + 3*7.5 = 47.5, while a small dog (expectancy 14) reads 25 + 3*5.5 = 41.5. Big dogs enter senior territory sooner, which the simple 7x rule hides.