Age calculator

How age is counted

In most of the world, you are zero when you are born and you gain a year on each birthday. So your age in years is the number of birthdays you have passed. That part is simple. The harder part is the exact figure in years, months and days, because months differ in length and there is more than one way to break the remainder down.

This calculator works in three steps. First it finds your most recent birthday. Next it steps forward whole months from that birthday for as long as it does not pass the target date. Whatever remains is the day count. If the month step lands on a date that doesn’t exist, such as April 31, it uses the last day of that month. That is the method used by most software and it matches how people talk: someone born March 31 is “one month older” on April 30.

Worked examples

Calculators can honestly disagree by a day or two on the months-and-days part, since some subtract days first and borrow from the previous month, and that gives a different remainder near a month’s end. The years figure and the total days are never in doubt. If two numbers differ, the total in days is the one to check.

Leap-day birthdays

People born on February 29 have a real birthday only in leap years, which come every four years with exceptions at some century years: 2000 was a leap year and 2100 will not be. In the other years they celebrate on February 28 or March 1. This tool counts the birthday as reached on February 28 in a common year, the same result as adding whole years with the month-end rule. Legal rules are not uniform. A jurisdiction can treat a person born on February 29 as reaching a legal age on March 1 in a common year, so for anything official, such as voting, drinking or licensing, check the local rule. The difference is only ever one day. The result shows both the count and your next real February 29.

Which time zone and which time of day?

Age is usually counted in calendar dates, without the time of birth. Someone born late in the evening becomes a year older at midnight on their birthday, in the place they are at that moment, and this tool uses your device’s time zone for today’s date. If you were born in another time zone, the official date is the one on your birth record, and that is the date to enter. Do not convert it.

Other ways of counting age

Traditional East Asian reckoning starts at one at birth and adds a year at the new year, so it can read one or two years above the international figure. Insurers may use “age nearest birthday”, which rounds to the closest one. Schools often use age on a fixed cutoff date, which you can check with the “Age on” box. Whenever a form states its own rule, follow it. This page always gives the ordinary completed-years figure.

Milestones in days

Counting in days gives a set of milestones you won’t find on a birthday card. Your 10,000th day comes when you are 27, and your 20,000th when you are 54. The table in the results gives those dates for your own birth date, since leap days and the start date shift them. Each is simply the birth date plus that number of days, the same arithmetic as the date calculator, with the day of birth as day zero.

In a spreadsheet

=DATEDIF(A1,TODAY(),"Y") gives age in completed years. Microsoft warns that DATEDIF can return incorrect results in some scenarios and that its “MD” unit can give a negative number, zero or an inaccurate result, so avoid that unit for the leftover days. For the total days, use =TODAY()-A1. Dates before 1900 are not stored as real dates in Excel, and it counts 1900 as a leap year, so very old dates need another tool.

For the same calculation between any two dates, use days between dates. To count down to the next birthday on screen, use days until a date. To learn more about a date, such as its week number and day of the year, see date facts.