Dog Age From Birth Date: Exact Years and Months

Quick answer Count the whole calendar months from the birth date to today, then the leftover days; years = months ÷ 12, and age in years for charts = days ÷ 365.25. A dog born on 15 March 2020 is 6 years 7 months on 19 October 2026 (2,409 days, 6.6 years), about 47 human years on the medium size chart.

“How old is my dog?” sounds easy until you have to answer it exactly. Is it 6 or 7? Six and a half? Six years and seven months? The answer matters for vet visits, for food stage, for how the dog compares with a chart and for planning when it becomes a senior. This guide shows how to work out exact age from a birth date in years, months, weeks and days, how to convert it to human years with the two methods in our calculators and what to do with the result. It also covers the awkward cases: dogs under a year, dogs born on 29 February and dogs whose birthday you have to choose. The Dog Age from Birth Date Calculator does all of it.

Counting from a birth date

The calculator does two things. It counts days between the birth date and the as-of date, and it counts whole calendar months plus leftover days.

Quantity	How it is worked out
Days	Number of days from birth to the as-of date
Weeks and days	Days ÷ 7, remainder as days
Whole months	Calendar months completed; if the day of the month has not yet been reached, subtract one
Years and months	Months ÷ 12 as whole years, remainder as months
Age in years for charts	Days ÷ 365.25

The divisor 365.25 allows for leap years. The two ways of describing age, calendar years and months and decimal years, are close but not identical, and both are shown.

Worked examples as of 19 October 2026

Birth date	Days	Weeks (and days)	Years and months	Decimal years
15 March 2020	2,409	344 w 1 d	6 years 7 months	6.59
1 September 2023	1,144	163 w 3 d	3 years 1 month	3.13
4 July 2018	3,029	432 w 5 d	8 years 3 months	8.29
20 December 2025	303	43 w 2 d	0 years 9 months	0.83
9 August 2026	71	10 w 1 d	0 years 2 months	0.19

Take the first: from 15 March 2020 to 15 October 2026 is 6 years and 7 months (the 15th of October has passed), and from 15 October to 19 October is 4 more days. The total in days is 2,409, and 2,409 ÷ 365.25 = 6.59 years. A dog with that birthday is 6 years, 7 months and 4 days old.

Converting to human years: two methods

Our calculators show two ways of putting a dog’s age in human terms.

1. The size chart. The first dog year counts as about 15 human years, the second as 9 more (24 at two years), and each later year adds 4, 5, 6 or 7 for small, medium, large and giant dogs. For fractions of a year, the calculator scales linearly.

2. The epigenetic formula. Human age = 16 × ln(dog age) + 31, from a 2020 study of DNA methylation in Labradors (Wang and colleagues, Cell Systems). It applies to dogs of one year or more, and takes no account of size.

Dog’s age	Small	Medium	Large	Giant	Epigenetic formula
0.83 years (9 months)	12.4	12.4	12.4	12.4	n/a
3.13 years	28.5	29.6	30.8	31.9	49.3
6.59 years	42.4	47.0	51.5	56.1	61.2
8.29 years	49.2	55.4	61.7	68.0	64.8

The columns show how much the methods differ. For the 3-year-old, the chart gives about 29 to 32 human years and the formula 49. At 8.3 years, the medium chart gives 55 and the formula 65. The formula was fitted to a particular breed over a limited age range, and it reflects biology rather than a size-specific lifespan. Neither is a measurement of the dog’s health; they are ways to think about life stage. Our guide to dog years to human years shows a full chart.

Life stage from the age

AAHA’s 2019 Canine Life Stage Guidelines describe puppy, young adult, mature adult, senior and end of life, with the senior stage roughly the last 25% of expected lifespan. Use the exact age with the Dog Life Stage Calculator and our guide to dog life stages. For example, a 6.6-year-old Labrador-size dog with an expected lifespan of 12 years reaches the senior stage at 9, so it is a mature adult now, with 2.4 years to go.

Puppies and the first year

For a dog under a year, years are too coarse. A dog of 0.83 years is 9 months, and the chart gives 15 × 0.83 = 12.4 human years. For puppies, weeks matter: our guides to puppy age in weeks and how old is my puppy show how to count and what changes by age. The epigenetic formula is not defined below one year (the logarithm of a number less than 1 is negative and the formula is not meant for that range).

Leap days and tricky dates

A dog born on 29 February celebrates in common years on 28 February or 1 March; the calculator uses 28 February in non-leap years. For counting days, leap years are handled automatically. For month counting, if the birth day does not exist in a later month (for example the 31st in a 30-day month), the calculator uses the last day of that month. It does not change the result by more than a day.

If you do not know the birth date

For a rescue dog, ask the rescue, then your vet. A vet estimates age from teeth, coat, eyes and general condition, usually as a range. Choose a notional birth date, such as the adoption date minus the estimated age, and use it consistently. Our guide to how to tell a dog’s age explains what the vet looks at, and how to pick a birthday for a rescue pet gives practical advice.

What to do with the number

Vet planning. Age tells you which checks and vaccines are due; your vet sets the plan.
Food. Puppies use age in weeks for the energy factor; adults use weight and neutering status. See puppy feeding chart and how much to feed a dog.
Life stage. Use the stage to plan check-ups and home changes; see when a dog is senior.
Records. Keep birth date, size group and the date of the last calculation with your pet’s papers.
Birthdays. The calculator can tell you the next birthday and the days to go.

A worked case

Bella was born on 4 July 2018. She is a Labrador-size dog, so the size group is large. On 19 October 2026 she is 3,029 days old, or 8 years and 3 months (8.29 years). On the large chart, she is 24 + 6 × 6.29 = 61.7 human years. On the epigenetic formula, she is 16 × ln(8.29) + 31 = 64.8. For planning, her owner uses the range 62 to 65, which puts her in the later part of the mature adult stage. With an expected lifespan of 12 years, the senior stage begins at 9, which is 0.7 years away, so Bella’s owner books a baseline check with the vet.

Tracking age over time

Update the calculation on each birthday and at each vet visit. A small table keeps it simple:

Date	Age (years and months)	Decimal years	Human years (chart)	Notes

For a puppy, update monthly; for an adult, yearly; for a senior, at each check-up.

Counting across month ends

The part that trips people up is a birth date near the end of a month. The simplest rule is to count whole months from the birth date to the same day number in later months, and then count the leftover days. A dog born on 15 March 2025 is, on 19 October 2026, 1 year, 7 months and 4 days old: 15 March 2026 is the first birthday, 15 October 2026 is 7 months later and 19 October is 4 days after that. In days that is 365 + 214 + 4 = 583, or about 19.2 months.

Born	On 19 Oct 2026	Whole months	Leftover days
15 Mar 2025	1 y 7 m 4 d	19	4
19 Oct 2025	1 y 0 m 0 d	12	0
1 Jan 2026	0 y 9 m 18 d	9	18

If the birth day does not exist in a month (for example 31 January into February), pick one rule, such as using the last day of the month, and use it every time. A dog born on 29 February is easiest to handle by treating 28 February as the birthday in common years. The age in days does not depend on any of these choices, which is why the chart conversion uses days.

Common mistakes

Mixing calendar years with decimal years.
Using the wrong size group for the chart.
Applying the epigenetic formula under one year.
Forgetting that both methods are approximations.
Counting from the adoption date.
Not updating after a birthday.
Treating human years as a health measure.

Quick summary

Count days and whole calendar months from the birth date; decimal years = days ÷ 365.25.
A dog born on 15 March 2020 is 6 years 7 months (2,409 days) on 19 October 2026.
Human years by size chart: 15, 24, then +4/+5/+6/+7 a year; epigenetic formula 16 × ln(age) + 31.
The two methods differ (47 against 61 for a 6.6-year-old medium dog), so treat them as a range.
Use exact age for life stage, vet planning and records.
Use the Dog Age from Birth Date Calculator.

Frequently asked questions

How do I work out my dog's exact age?

Count whole calendar months from the birth date, then the extra days. Divide months by 12 for years and months.

How many years is 2,409 days?

2,409 ÷ 365.25 = 6.6 years, which is 6 years and 7 months.

How do I convert it to human years?

Use the size chart (15 for year one, 24 for year two, then +4, +5, +6 or +7 a year by size) or the epigenetic formula 16 × ln(age) + 31 for dogs over one year.

Why do the two methods differ?

The size chart varies with size and is an approximation; the formula was derived from Labradors and ignores size. Both are approximations.

What if I do not know the birth date?

Use your vet's estimate and a notional date, then record it. See how to tell a dog's age.

Do I use years or months for puppies?

Use weeks and months for puppies, since they change quickly.

Sources

Wang T. et al. (2020) Quantitative translation of dog-to-human aging by conserved remodeling of the DNA methylome. Cell Systems
AAHA (2019) Canine Life Stage Guidelines. Journal of the American Animal Hospital Association 55:267-290
Size-based human-year chart: commonly published rule of thumb (an approximation, not taken from the AAHA life-stage guidelines)

Reviewed by the CalcuPaw Veterinary Review Team

Last reviewed 19 Oct 2026 · Meet the review team · This guide is general information, not veterinary advice.