AAHA’s size chart and the Labrador methylation formula give different estimates, not an exact medical age.
This calculator uses the named AAHA / Zoetis Canine and Feline Age Chart, adapted from Dog Owner’s Home Veterinary Handbook. The table is an approximate physiological-age comparison, not a lifespan prediction or your dog’s medical age. Adult size is a broad guide; breed, genetics, health and individual development also matter. All ages 1–16 below match that poster, rather than combining values from different charts.
The source groups dogs by adult weight: up to 20 lb, 21–50 lb, 51–90 lb and over 90 lb. Its rounded metric equivalents are up to 9.07 kg, 9.52–22.7 kg, 23.1–40.8 kg and over 40.8 kg. Those printed whole-pound ranges leave rounding gaps: for fractional weights, use up to 20, over 20 through 50, over 50 through 90 and over 90 lb. One pound is 0.45359237 kg; 90 lb is about 40.82 kg. A 95 lb dog belongs in Giant, not Large.
Fractional ages are linearly interpolated between yearly rows. Under one year, this tool scales the first-year value; above 16, it extends the final interval. These are calculator assumptions, not additional published chart values. A life-stage label based on the human-equivalent number is a rough display category, not a veterinary life-stage assessment.
At 13 dog years, the AAHA / Zoetis chart gives approximately 68 human years for Small, 74 for Medium, 82 for Large and 101 for Giant. Multiplying by seven gives 91 for every dog, which loses the size differences. The numbers are comparisons, not a diagnosis or a claim about remaining lifespan.
Wang and colleagues compared age-related DNA methylation patterns in 104 Labrador retrievers with human data. Their 2019 preprint was published in Cell Systems in 2020. The fitted relationship is human age = 16 × ln(dog age in years) + 31. This calculator evaluates that equation; it does not collect or test your dog’s DNA.
The formula gives 31 at one dog year and about 72.04 at 13, regardless of the selected size. The chart gives 15 at one year for every size, and 68–101 at 13. The methods measure different comparisons: a broad physiological chart versus a Labrador-derived methylation model. The research number is not a more accurate individual age or a replacement for the size chart, and it is not validated here across all breeds. Do not average the two to invent an exact age.
Use the result to start a conversation with your veterinarian, not to select treatment or a screening schedule automatically. Actual age, adult weight, breed history, mobility, appetite and medical findings matter more than a human-year label. Ask your vet what preventive care suits your individual dog.
Measure age consistently: enter completed years plus extra months, so six months is 0 years and 6 months. Pick the adult weight class rather than a puppy’s present weight. Research formulas and size charts cannot predict how long a particular dog will live.
13-year-old medium dog: The Medium chart row at 13 is 74. The Labrador equation gives 16 × ln(13) + 31 = 72.04, displayed rounded to 72. Similarity here does not make the methods interchangeable.
13-year-old, 95 lb dog: 95 lb exceeds the poster’s 90 lb boundary, so choose Giant. The chart estimates 101 human years; the research equation still gives about 72 because it does not adjust for weight.
Use the existing [Dog Age Calculator](/tools/health/dog-age-calculator) for the full sourced table and an estimate by adult size.
[AAHA / Zoetis — Canine and feline age chart (adapted from Dog Owner’s Home Veterinary Handbook)](https://www.aaha.org/wp-content/uploads/globalassets/02-guidelines/canine-life-stage-2019/canine_and_feline_age_chart_poster.pdf)
[Wang et al. — DNA methylome aging study, Cell Systems (2020; 2019 preprint)](https://pmc.ncbi.nlm.nih.gov/articles/PMC7484147/)