Age Calculator

Age is counted in calendar units, not by dividing days by 365. The calculator finds how many whole months have passed since the date of birth, where a month counts once its anniversary day has arrived, splits them into years and months, and then counts the days since the last monthly anniversary. Both dates are read as midnight UTC, so a time zone or a clock change can never add or remove a day.

whole months = months from the date of birth to today, less one if this month’s anniversary is still ahead years = whole months ÷ 12, whole part · months = the remainder days = days since the last monthly anniversary

Someone born on 15 March 1990 and measured on 26 September 2026 has lived 438 whole months, which is 36 years and 6 months, plus the 11 days since the anniversary on 15 September: 36 years, 6 months and 11 days, or 13,344 days in all.

A year is not a fixed number of days. Dividing a day count by 365 ignores leap days, and about ten of them pass before a 40th birthday: someone born on 26 September 1986 has lived 14,600 days on 16 September 2026, and 14,600 ÷ 365 is exactly 40, ten days before they turn 40. Dividing by 365.25 averages the leap days in and is right most of the time, but not on every birthday: someone born on 1 March 2000 has lived 1,095 days on their third birthday, and 1,095 ÷ 365.25 is only 2.998. Counting calendar anniversaries is exact on every date.

A leap-day birthday has a real anniversary only one year in four. In the other years this calculator counts the birthday on 28 February, the last day of the month, exactly as it treats every month-end: someone born on 31 January completes a month on 28 or 29 February. The law does not always agree. Hong Kong, for one, counts a 29 February birthday on 1 March in common years, so where a legal age matters, for a licence or a pension, check the local rule.

Traditional East Asian reckoning counts a newborn as one year old and adds a year at each new year rather than on the birthday, so a baby born on 31 December turns two the next day. South Korea made international age, which starts at 0 and adds a year on each birthday, the standard in law and administration on 28 June 2023, although a few laws, such as the one on military service, still count by calendar year. Japan made the switch with a law that took effect in 1950, and in China the traditional count, xusui, survives mainly in family use. This calculator always gives international age.

• Subtracting the years only. 2026 − 1990 gives 36 for the whole of 2026, but someone born in December is still 35 until their birthday.
• Dividing days by 365. The leap days pile up, and by 40 the result turns over about ten days early.
• Mixing up time zones. A birth just before midnight in one time zone happened on the next date in another. Use the date on the birth certificate.
• Forgetting the leap-day rule. A 29 February birthday falls on 28 February here in common years, and on 1 March under some laws.