How Age Is Calculated (Years, Months, and Days)
Age looks simple — subtract your birth year from this year — but that quick sum is often wrong by a year, because it ignores whether your birthday has happened yet. The proper method counts completed units: whole years first, then the leftover whole months, then the remaining days.
The basic method
To find an exact age, you count in three steps: the number of complete years from the date of birth to today, then the whole months since the most recent birthday, then the leftover days. So someone born on 10 March 2000, measured on 25 June 2024, is 24 years, 3 months, and 15 days old — not simply "24" and not "2024 − 2000 = 24" by luck.
Why you stay a whole number until your birthday
Age counts completed years, so you are 24 for the entire year between your 24th and 25th birthdays and only tick over to 25 on the day itself. This is why subtracting birth year from current year can be off by one: it silently assumes your birthday has already passed this year, which for half the year it has not.
Month-end and leap-day birthdays
Two edge cases need care. When a birthday is on the 31st and the current month is shorter, the month count clamps to the month's last day so it never overflows. And a 29 February birthday is handled by treating it as the end of February in common years: someone born on 29 February 2004 is exactly 20 on 29 February 2024, and turns 20 on 28 February in years that have no leap day. Our age calculator and birthday calculator both use this month-clamped method.
Age in weeks, months, hours, and total days
Years-months-days is the everyday form, but the same span can be expressed other ways. Total days are useful for milestones (your 10,000th day is around 27 years), weeks are common for infants, and months for young children. Because every count is built from real calendar days, leap years are already included. The age in weeks tool gives these alternative views.
Different conventions around the world
The method above is the international standard, but it is not the only one. Traditional East Asian age reckoning counted a person as one year old at birth and added a year at each New Year rather than on the birthday, which could make someone one or two years "older" than their international age. Most official systems today use the completed-years method described here.