How to calculate age in years, months, and days
Why “divide days by 365” is wrong, how leftover months work, and what happens to a 29 February birthday.
Do not divide total days by 365
Leap years make a year 365 or 366 days, so “days lived ÷ 365” drifts. The reliable method is calendar arithmetic: subtract the birth year, then borrow a year if the birthday has not occurred yet this year, and do the same for months and leftover days.
Example: born 20 March 2000, today 10 February 2026. Years start at 26, but 10 February is before 20 March, so you are 25 years, 10 months, and some days — not 26.
Leap-day birthdays
People born on 29 February legally age on 28 February in common years in most civil systems. An age calculator that uses the local calendar date will match that. If you need a party on 1 March instead, that is a social choice, not a different age.
Try related tools
- Age calculator — Age in years, months, and days, plus days until the next birthday.
- Date difference — How many days, weeks, and hours between two dates.