How to calculate age from a date of birth

The formula for age in years, months and days, worked through by hand, the cases where it trips people up (the 31st, February 29), the Excel formula and its known flaw, and how Korean age differs.

Oct 19, 2026 · By Calcelate Team

The short version: subtract the birth year from the current year, then subtract 1 more if the birthday has not come yet this year. That gives age in whole years, which is what almost every form asks for. Age in years, months and days takes two more steps, and those steps are where most mistakes happen.

What is the formula to calculate age?

For whole years:

age = current year − birth year, minus 1 if this year’s birthday is still ahead

Someone born on June 15, 1990 is 36 on June 15, 2026. On June 14, 2026 they are still 35, because the birthday is one day away.

For an exact age, count in three steps and keep the order:

1. Whole years. Add years to the birth date for as long as you do not pass the date you are checking.
2. Whole months. From there, add months the same way.
3. Days. Count the days that are left.

Born June 15, 1990, checked on September 14, 2026: adding 36 years reaches June 15, 2026. Adding 2 months reaches August 15. From August 15 to September 14 is 30 days. The age is 36 years, 2 months and 30 days. The plain day count between the two dates is 13,240 days, or 1,891 full weeks.

The age calculator shows these same three steps for any pair of dates, in a block under the result.

How can I calculate my age from birth?

The method above works for most dates. It needs care in two places: when the birth day is a day that the current month does not have, and when the birthday is February 29.

Born on the 31st. A month is complete when the same day of the month comes around again. February has no 31st, so a baby born on January 31 is 28 days old on February 28, not one month. On March 1 the baby is 1 month and 1 day old: the month ends on the last day of February, and one more day has passed.

Born on February 29. In a year without February 29, the person has not finished another year on February 28. They are 26 years, 11 months and 30 days old that day, and turn 27 on March 1. That is also what the calculator uses. Some laws write this down: in Hong Kong, for example, the law sets March 1 as the birthday in non-leap years. Rules differ by country, so check the local one when the date decides something legal, such as a driving test or a pension.

How to calculate age in dd mm yyyy?

The arithmetic does not change with the date format. What changes is how you read the date. In the US, 03/04/2010 is March 4. In the UK, most of Europe and India, the same digits mean 3 April. Before counting, write both dates out with the month as a word. That removes a common error: reading the day as the month.

Is there a formula in Excel to calculate age?

Yes, the DATEDIF function. With the date of birth in cell A2:

• =DATEDIF(A2,TODAY(),"Y") gives complete years.
• =DATEDIF(A2,TODAY(),"YM") gives the months after the last birthday.

Replace TODAY() with a cell reference to get the age on another date. Google Sheets has the same function with the same units.

For the days, there is an “MD” option, but Microsoft does not recommend it: it “may result in a negative number, a zero, or an inaccurate result.” If the days matter, check them in a calculator.

Korean age and other ways of counting

Not every country has always counted age from the birthday. In South Korea, the traditional “Korean age” made a baby 1 year old at birth and added a year to everyone on January 1. A baby born on December 31 was 2 years old the next day. Since June 28, 2023, the law requires the international method, counted from the date of birth, for official use. The Law Library of Congress explains the change and notes that some laws, such as the Military Service Act, count by birth year alone.

Age in months for babies

Doctors use months for babies and young children: growth charts and vaccine schedules are set by age in months. “18 months” means 18 calendar months from the birth date. It is not 78 weeks, because 78 weeks is 546 days and 18 months is 546 to 550 days, depending on which months they cover. Count months the same way as above. For gaps between any two dates that are not birthdays, use the date calculator.