How to calculate an exact age in years, months and days

Working out someone's age in whole years is easy. Working it out exactly — 25 years, 5 months and 21 days — is surprisingly fiddly, because months are different lengths and leap years come and go. Forms, contracts and eligibility rules often depend on getting it exactly right.

Why dividing days by 365 goes wrong

A common shortcut is to count the days between two dates and divide by 365, or by 365.25 to allow for leap years. Both drift. Near a birthday, the shortcut can be a day out either way — exactly the situation where eligibility for a school place, a pension or a licence is decided. Real age is counted on the calendar, not by averaging.

How age is counted properly

1. Whole years: subtract the birth year from the current year, then take one off if this year's birthday has not happened yet.
2. Whole months: count the months since the last birthday, taking one off if the day of the month has not been reached yet.
3. Days: count the days left over, borrowing from the length of the previous calendar month when needed.

Borrowing from the real previous month is what makes the result correct across months of different lengths. Someone born on 31 January is one month old on 28 February in a common year, because February has no 31st.

A worked example

Someone born on 15 March 2001, checked on 5 September 2026:

• The 2026 birthday has passed, so 2026 − 2001 = 25 years.
• From 15 March to 15 August is 5 months.
• From 15 August to 5 September is 21 days.

The Age Calculator gives the same answer — 25 years, 5 months and 21 days — together with the totals: 9,305 days, the equivalent in weeks, months and hours, and a countdown of 191 days to the next birthday.

Birthdays on 29 February

A person born on 29 February has a real birthday only once every four years. In other years, the most widely used convention treats their birthday as 1 March, so they turn a year older on 1 March. That is how the calculator counts. Some laws and organisations use 28 February instead, so when an exact legal date matters — a licence, a contract, a pension — check the rule that applies where you are.

Age on a particular date

Many rules ask how old someone is on a specific day rather than today: “age on 1 September” for school entry, “age on the closing date” for a job or a competition, “age at the start of the policy” for insurance. Set the second date in the calculator to that day instead of leaving it as today.

The same works for any two dates — the length of a tenancy, the time since an event, the age of a building — because counting the gap between two dates is the same calculation.

Does the day of birth count?

By the usual convention, a person is zero days old on the day they are born and one year old on their first birthday. The day of birth is the starting point, not day one. That is also how the totals are counted.

Totals in days, weeks and hours

Ages in days or weeks are useful for babies, for milestones such as “10,000 days old”, and for anything measured in a fixed unit. These are counted from the real calendar too. For converting a duration between units without specific dates — how many hours are in 90 days — use the Time Converter, which works with average month and year lengths.

Other ways of counting age

Not every tradition counts the same way. Some East Asian traditions count a newborn as one year old; South Korea moved its official age counting to the international method in 2023. When a form asks for your age, it means the international method described here unless it says otherwise. For pets, see how dog and cat years really compare.