Method and practical guidance
A pet’s “human age” is an analogy, not a clinical measure of health. This tool assigns dogs 15 human-equivalent years at one year and 24 at two, then adds an illustrative 4, 5, 6, or 7 per year for small, medium, large, and giant dogs. Cats use 15 at one year, 24 at two, then 4 per year. These familiar rules are simplified: Animal Health Associates’ AAFP/IDEXX-based chart shows that dog size matters and its actual tabulated values differ from a single formula. We do not claim these slopes reproduce that chart or predict an individual animal’s biological age.
For horses we use illustrative anchors of 0, 6, 12, and 18 human-equivalent years at horse ages 0, 1, 2, and 3; thereafter we add about 2.5 per year. The Horse’s equine age relationship report describes slower change of about 2.5 per year from age four. The first three anchors and interpolation are our explanatory approximation, not a veterinary conversion scale.
Fractional ages are linearly interpolated between early anchors; after the anchors, the stated yearly increments are continuous. Expected adult dog size is entered separately; small is up to 20 lb, medium over 20–50 lb, large over 50–90 lb, and giant over 90 lb. Individual breed, condition and health affect aging far more than a simple number. Ask your veterinarian about life-stage care. For a different pet estimate, try the dog size predictor or explore all pet tools.
Frequently asked questions
Is one dog year really seven human years?
No. Dogs develop rapidly in their early years, and larger dogs often age differently from small dogs. The result here is only an illustrative comparison.
How is a half-year age handled?
The calculator interpolates linearly between early-year anchors and uses a continuous yearly rate afterward; a half year is not rounded to a whole year.
Does the horse number tell me when my horse is senior?
No. A numerical human-year analogy cannot establish health or senior status. Your equine veterinarian can evaluate the horse's individual condition.