How to calculate exact age
Exact age is counted on the calendar: whole years to the last birthday, then whole months, then days. This guide walks through the method with an example and explains the two edge cases that trip people up - 29 February birthdays and the 31st of a month.
The short version
- Count whole years to the most recent birthday on or before the date.
- Count whole months from that birthday.
- Count the remaining days. On the calendar, not by an average.
- For the next birthday, find the next occurrence of the birth month and day after the date.
Skip the arithmetic. The free Age Calculator does this for you, shows the formula and the working with your own numbers, and runs in your browser - nothing you type is uploaded. Also useful: Hours Calculator.
A worked example
Born 15 January 2000. Age on 7 October 2026:
Why not divide by 365?
Dividing days by 365 (or 365.25) gives an approximation, and it drifts: 26 × 365 = 9,490 days misses the leap days. The calendar method always matches how people actually count birthdays.
When the birth day does not exist in a month
A month is complete when the same day of the month comes round again. If a month is too short, its last day counts instead. Born 31 January: on 29 February 2000 exactly one month has passed, because 29 February is the last day of that month.
The 29 February rule
People born on 29 February have no birthday in three years out of four. There is no single universal rule: some places treat 28 February as the birthday in those years and others 1 March. State which one you are using. Under the 28 February rule someone born on 29 February 2000 turns 1 on 28 February 2001; under the 1 March rule they turn 1 on 1 March 2001.
Mistakes to avoid
- Using 30 days for every month.
- Forgetting leap years when counting days.
- Counting the birth day itself twice. Age on the birth date is 0.