How old is an 18-year-old dog in human years?

The AVMA chart stops at 16 years, where a medium-size dog is 87 in human years, so an 18-year-old dog is older than that on the chart’s scale. The 2020 DNA-clock formula gives about 77, but the oldest dog in that study was 16 and the formula runs lower than the chart in old age.

Patient recordDog
Age
18 years
Size
Medium, 20 to 50 lb

Human age

77years

DNA-clock formula (the AVMA chart stops at 16 years)

AVMA chartmedium dogends at 16 yr
“Times 7” rulewhy it’s wrong126

Past 16 the AVMA chart has no row, so this is the formula alone. The oldest dog in the DNA study was 16, so treat the formula as a rough guide here.

Life stage

  • Puppy
  • Young adult
  • Mature adult
  • Senior (this stage)

Medium dogs are senior from about 8 to 10 years (AVMA; stages from AAHA).

The card shows a medium-size dog. For months or another size, use the dog age calculator.

An 18-year-old dog by size

AVMA publishes a year-by-year human age only for medium dogs. What size does change is when a dog becomes a senior, so here is the life stage at 18 years for each AVMA size group.

Size and adult weightLife stage at 18 yearsSenior from (years)
Small or toyless than 20 lbSenior8 to 11
Medium20 to 50 lbSenior8 to 10
Large50 to 90 lbSenior8 to 9
Giantmore than 90 lbSenior6 to 7

The math

AVMA chart: no row past 16 yearsThe last row is 16 years = 87 human years, for a medium-size dog.

DNA clock: 16 × ln(18) + 31 = 16 × 2.890 + 31 = 77.2ln is the natural logarithm. Rounded, that is 77 human years.

The formula comes from DNA methylation in Labrador retrievers; its authors say breeds with longer or shorter lives could follow a different curve. The oldest dog in the study was 16.

Nearby ages

Dog ageAVMA chart (medium)DNA clockTimes 7
14 years787398
15 years8374105
16 years8775112
17 years—76119
18 years—77126
19 years—78133
20 years—79140

See all ages in the dog years chart, or compare an 18-year-old cat.

Sources

  1. AVMA, “Pets age faster than people” (PDF). Dog column for a medium-size dog of 21 to 50 lb, ages 1 to 16; cat column, ages 1 to 16. AVMA adds that pet ages are approximate and that weight and breed may influence aging.
  2. AVMA, “Caring for senior cats and dogs.” Dog size groups by weight, the age each group becomes senior, cats senior after about 10, and the statement that dogs don’t age 7 human years per year.
  3. AVMA “Senior Pets” client brochure, revision 7/10, since replaced by a 2023 edition without the chart (archived copy). Human-age ranges from small to very large dogs at 7, 10, 15 and 20 years.
  4. Wang T, Ma J, Hogan AN, et al. “Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome.” Cell Systems 11 (2020). The formula human age = 16 ln(dog age) + 31, from 104 dogs, mostly Labrador retrievers, aged a few weeks to 16 years. See also the UC San Diego news release.
  5. UC Davis School of Veterinary Medicine, “Calculating your dog’s age.” Why the times-7 rule is inaccurate and where it likely came from.
  6. 2019 AAHA Canine Life Stage Guidelines (PDF). Puppy, young adult, mature adult, senior and end-of-life stages.