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Dog Age from Birth Date Calculator

Enter your dog's birth date and size to see its exact age and its age in human years.

✔ Reviewed by a licensed veterinarianContent updated 18 Oct 2026Reviewed 18 Oct 2026Sources: Size-based human-year chart: commonly published rule of thumb · Wang T. et al.
Dog Age from Birth Date CalculatorFree · instant
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Quick answer. A medium dog born on 18 October 2021 is 5 years old on 18 October 2026. On the size chart that is about 39 human years; the 2020 epigenetic formula gives about 57. Both are approximations, so use them as a range.

How to use the dog age from birth date calculator

  1. Enter the birth date.
  2. Enter your dog's adult weight to pick the size class.
  3. Press Calculate.

Worked example

A medium-size dog (15 kg adult weight) born on 18 October 2021, checked on 18 October 2026.

Birth date
2021-10-18
On this date (leave empty for today)
2026-10-18
Adult weight
15 kg
39 human years
Age
5 years
Size class
medium
Size-chart estimate
39 human years
Epigenetic formula (Wang 2020)
56.7 human years

Both methods are estimates; dogs of different sizes age at different speeds.

Example values are illustrative; this result is produced by the same calculator above.

How we calculate it

age (years) = days since birth ÷ 365.25; size chart: year 1 ≈ 15, year 2 ≈ 24, then +4 / +5 / +6 / +7 per year (small / medium / large / giant); epigenetic: 16 × ln(age) + 31 (Wang 2020)

Sources: Size-based human-year chart: commonly published rule of thumb (an approximation, not taken from the AAHA life-stage guidelines) · Wang T. et al.

Understanding your result

Instead of typing the age, enter your dog's birth date. The calculator counts the days to today (or a date you choose) and divides by 365.25 to get the age in years, then converts it to human years.

It uses two methods side by side. The size chart gives about 15 human years for year one, 24 for year two and then 4, 5, 6 or 7 more per year for small, medium, large and giant dogs. The 2020 epigenetic formula, 16 × ln(age) + 31, comes from a study of Labradors.

A medium dog born on 18 October 2021 is 5 years old on 18 October 2026, about 39 human years on the chart and about 57 by the formula. Treat them as a range, not a precise number.

Common mistakes

  • Using the date you adopted the dog instead of its birth date.
  • Choosing the dog's current weight instead of its expected adult weight.
  • Treating one human-year figure as exact.
  • Using it for very young puppies; use the Puppy & Kitten Age Calculator.
  • Forgetting that mixed breeds can be hard to size.

Good to know

  • Large and giant dogs age faster in later life, so size matters more as a dog gets older.
  • Life stage matters for check-ups, diet and exercise; ask your vet what is right for your dog.
  • The Pet Birthday Calculator shows when the next birthday is.

Frequently asked questions

How do I find my dog's age from its birth date?

Count the days from the birth date to today and divide by 365.25. The calculator then converts the age into human years using a size chart and the 2020 epigenetic formula.

Why does my dog's size matter?

Large and giant dogs age faster in later life than small dogs, so the chart adds more human years per dog year for bigger sizes.

Which result should I trust?

Both are approximations. The chart reflects size; the epigenetic formula comes from Labradors. Treat them as a range.

What if I do not know the birth date?

Use your vet's or the rescue's estimate. The result is then an estimate as well.

Can I check the age on a different date?

Yes. Pick a date in the second box. Leave it empty to use today.

Is this different from the Dog Age Calculator?

It starts from a birth date rather than an age you type in. Both use the same conversion methods.

Reviewed by a licensed veterinarian
Last reviewed 18 Oct 2026 · How we review

Sources

  1. Size-based human-year chart: commonly published rule of thumb (an approximation, not taken from the AAHA life-stage guidelines)
  2. Wang T. et al. (2020) Quantitative translation of dog-to-human aging by conserved remodeling of the DNA methylome. Cell Systems 11:176–185
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Estimates only. Every pet is different — confirm with your veterinarian.