3 years in dog years is about 28 human years
A 3-year-old dog is about 28 in human years if it is medium-sized (21 to 50 lb), on AVMA’s chart. A small dog of the same age is about 28, a large one 29.5 and a giant one 31.5. The DNA clock of a 2020 study gives 48.6.
3 years in human years, by size
| Size | Human years | Life stage (AAHA) | Senior from (VCA) |
|---|---|---|---|
| Small (20 lb or less) | 28 | Young adult | 11 years |
| Medium (21 to 50 lb) | 28 | Young adult | 10 years |
| Large (51 to 100 lb) | 29.5 | Young adult | 8 years |
| Giant (over 100 lb) | 31.5 | Young adult | 7 years |
Dog age calculator, with 3 years filled in: change the size, or work a human age back to dog years.
The end of growing up
AAHA places the end of the young adult stage at the completion of physical and social maturation, which happens in most dogs by 3 to 4 years of age. A 3-year-old is near that line.
From here the sizes part. PetMD gives about 4 human years a year after 2 for a small dog, 5 to 6 for a large dog and 7 to 8 for a giant dog, so the third year already puts a giant dog three and a half human years ahead of a small one on this page’s chart.
What changes from 2 to 3
This birthday adds 4 for a small dog, 4 for a medium dog, 5.5 for a large dog and 7.5 for a giant dog in human years on the chart. The DNA clock adds 6.5.
Life stage at 3
Dogs of every size are young adults at 3, by the stages of the 2019 AAHA guidelines and the senior ages of VCA (small 11, medium 10, large 8, giant 7).
No size turns senior at exactly 3.
What the DNA clock says about a 3-year-old dog
Wang et al. (Cell Systems, 2020) matched the DNA methylation of 104 Labrador retrievers with that of people and fitted human age = 16 × ln(dog age) + 31. For 3 years that is 48.6 human years, 20.6 more than the 28 of AVMA’s chart for a medium dog.
At 3, the clock reads older than the chart of every size: it ages young dogs much faster than the charts do.
The seven-year rule at this age
Seven times 3 is 21. The chart puts a 3-year-old dog between 28 and 31.5 human years by size, and the medium dog at 28, so the rule is 7 human years too low for a medium dog. No veterinary source on this page uses it.
Frequently asked questions
How old is a 3-year-old dog in human years?
About 28 for a small dog (20 lb or less), 28 for a medium dog (21 to 50 lb), 29.5 for a large dog (51 to 100 lb) and 31.5 for a giant dog (over 100 lb), on the charts of AVMA, Merck and PetMD. Your veterinarian can tell you where your own dog is.
Is a 3-year-old dog a senior?
Not by size alone: VCA counts dogs as seniors from 7 (giant) to 11 or 12 (small), so a 3-year-old of any size is not there yet. AAHA defines a senior as a dog in the last 25% of its expected life.
Other ages in dog years
Dog age chart: every age and size in one table · Dog age calculator
Sources
- AVMA, Pets age faster than people (chart for a medium-sized dog of 21 to 50 lb)
- Merck Veterinary Manual, Description of Dogs (Antinoff, reviewed October 2026), Dog years versus human years
- PetMD, How Old Is My Dog in Human Years? (Coates, DVM, December 2024)
- Wang T, et al. Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome. Cell Systems 2020;11(2):176–185
- The same study, free full text on PubMed Central (PMC7484147)
- 2019 AAHA Canine Life Stage Guidelines (Creevy et al., JAAHA 2019;55:267–290), Table 1
- VCA Animal Hospitals, How Old is Old? Comparing Dog Age to Human Age (Weir, Llera, Buzhardt)