Age Calculator

Age is a calendar difference, not a quantity of time divided up afterwards. The calculator subtracts the years first, then the months, then the days; when the day of the month comes out negative it borrows the length of the preceding month and carries one month back. That borrowing step is why the "days" part of an age depends on which month you happened to pass through. From January 31 to March 1 is one month and one day, because it borrowed from a 28-day February. From March 31 to May 1 the same span reads one month and zero days, because it borrowed from a 30-day April. Nothing is wrong — the months genuinely are different lengths. It is also why age reaches a birthday on the date, not after a fixed number of days. You turn 30 on your birthday whether the preceding year had 365 days or 366.

This is the most common way to get an age slightly wrong, and it always errs in the same direction: too old. Take someone who has just turned 40. They have lived through roughly ten February 29ths, so their total days is about 40 × 365 + 10 = 14,610. Divide that by 365 and you get 40.03 years — around 40 years and 10 days, ten days that do not exist. Dividing by 365.25 lands much closer, but still not exactly, because leap days do not arrive at a smooth rate. The reliable method is the one above: count whole years on the calendar, then the leftover months, then the leftover days. Total days is a fine number to know — it is exact, and it is what you want for a "10,000 days alive" milestone — but it is not a shortcut to an age.

A year is a leap year if it is divisible by 4 — except that a year divisible by 100 is not a leap year, unless it is also divisible by 400. So 1996, 2004 and 2024 are leap years. 1900 was not. 2000 was, because 400 divides it evenly. 2100 will not be, and that one will break a lot of spreadsheets built by people who only learned the first half of the rule. The exception exists because the astronomical year is not exactly 365.25 days. Adding a day every four years overcorrects by a bit under a day per century, so the calendar drops three leap days every four hundred years. That leaves an average calendar year of 365.2425 days, close enough that the drift is on the order of a day in several thousand years.

Someone born on a leap day has a real birth date that exists on the calendar only once every four years — and, thanks to the century rule, occasionally with an eight-year gap, as happened between 1896 and 1904. In ordinary years, practice differs on which day counts as the birthday. Some organizations treat February 28 as the anniversary, others March 1, and the answer can vary between a driver licensing agency, an insurer and a school in the same state. This calculator does not pick a side: it reports the calendar difference from the actual birth date, so on February 28 of a common year a leap-day child shows as not quite having completed the year, and on March 1 as having completed it. If the answer has legal or financial consequences, check the rule of the specific institution.

Alongside years, the result gives the total months, weeks and days lived. Total months is how infancy is normally tracked — a child of 1 year and 3 months is also 15 months, and pediatric growth charts and vaccination schedules are keyed to that number rather than to years. The reference date defaults to today but is fully editable, which turns the same tool into two others. Set it forward to answer "how old will she be at the wedding". Set it back to answer "how old was he when that happened". Set it to a cutoff date to check an age requirement — many programs and competitions are keyed to age as of a specific date rather than age today, and the two can differ by a year.