July 13, 2026 7 min read

How to Calculate Age From a Date of Birth

To calculate age, subtract the birth year from the current year, then take off 1 if the birthday has not happened yet this year. Someone born on November 25, 1990 is 35 on July 13, 2026, because 2026 minus 1990 is 36 and their November birthday is still ahead. That covers most forms. For an exact age in months and days, or in days lived, you need a little more care.

The quick method

Age, in the way it is counted in most of the world, is the number of birthdays you have completed. You are 0 until your first birthday, and you gain a year on each anniversary of your birth date. So the calculation has two steps:

1. Subtract the birth year from the current year.
2. Compare month and day. If today is before the birthday in the calendar, subtract 1.

Born March 4, 1988, checked on July 13, 2026: 2026 minus 1988 is 38, and March 4 has passed, so the age is 38. Born November 25, 1990: 36, minus 1 because November 25 has not arrived, so 35. Most mistakes come from skipping the second step, which is why “how old am I if I was born in 1990” has two answers depending on the date.

Exact age in years, months and days

For an exact age, step forward from the birth date in whole units and count what is left. This is the anniversary method, and it is the one to trust because it never produces negative numbers:

1. Add whole years to the birth date for as long as the result does not pass today. That count is the age in years.
2. From that last birthday, add whole months for as long as the result does not pass today.
3. Count the days from that point to today.

Worked example, born November 25, 1990, on July 13, 2026. Adding 35 years reaches November 25, 2025 (36 would overshoot). Adding 7 months reaches June 25, 2026. From June 25 to July 13 is 18 days. The exact age is 35 years, 7 months and 18 days.

On paper, people often use the borrowing method instead, like subtraction with carrying. Write the dates as year, month, day and subtract each column. If the days come out negative, borrow the number of days in the month before the current one, and take 1 off the months. If the months come out negative, add 12 and take 1 off the years.

Both methods agree here. They part ways near month ends, which is the first edge case below.

Age in days, weeks and hours

Age in days is the number of days from the birth date to today, not counting the birth date itself: on the day you are born you are 0 days old, and the next day 1. For the example above, November 25, 1990 to July 13, 2026 is 13,014 days. Leap days are what make this hard by hand, so count it with the date calculator or read our guide on how to count the days between two dates.

Weeks and months follow from that. Divide the days by 7 for weeks: 13,014 days is 1,859 weeks and 1 day. Total months come from the exact age: 35 years and 7 months is 427 months.

Hours are the least exact figure. Multiplying days by 24 gives 312,336 hours, but that assumes you were born at midnight and ignores daylight saving time, which shortens one day each spring and lengthens one each fall where it is used. Over a lifetime those roughly cancel, leaving at most an hour of error, but not knowing the birth time leaves up to 24. Treat an age in hours as a round number.

Edge cases

Born on February 29

February 29 exists only in leap years: every year divisible by 4, except century years not divisible by 400. Someone born on February 29, 2000 had their sixth real birthday in 2024 and will have the next in 2028 (the full rule is in leap year rules). In the three common years between, there are two conventions for the birthday:

• February 28, which keeps the birthday in February. The anniversary method below lands here too, because it falls back to the last day of a short month.
• March 1, the day after February 28. This is where the borrowing method lands: on February 28, 2026, the borrowing method gives 25 years, 11 months and 30 days, and on March 1 it gives exactly 26 years.

For legal ages, such as when a leapling can drive, vote or buy alcohol, the answer depends on the jurisdiction. Check the rule that applies to you rather than a calculator.

Born on the 31st

Month arithmetic breaks when the birth day does not exist in the target month. Born January 31, 2000: what is one month after January 31, 2026? As with February 29, the anniversary method uses the last day of the shorter month, so February 28, 2026 is 26 years and 1 month, and March 1 is 26 years, 1 month and 1 day. Naive borrowing gets this wrong. On March 1 it subtracts 31 from 1 and borrows only February’s 28 days, leaving minus 2 days. Spreadsheets have the same trap: Microsoft warns that the “MD” option of Excel’s DATEDIF function can return a negative number, a zero or an inaccurate result. Our guide to calculating age in Excel gives formulas that avoid it.

Born in another time zone

Your birth date is the local date where you were born, the one on your birth certificate. A baby born at 11:30 p.m. on December 31 in New York was born at 4:30 a.m. on January 1 in London time, but the birthday is still December 31. Do not convert it when you move.

Age systems around the world

Everything above is international age: zero at birth, plus one on each birthday. It is not the only system. In South Korea’s traditional count, a baby was 1 at birth and everyone gained a year on January 1, so a child born on December 31 turned 2 the next day.

South Korea switched its legal and administrative documents to international age on June 28, 2023, which made most people a year or two younger on paper overnight. At the switch, the government kept a third system, “year age” (the current year minus the birth year), for areas such as the school year, military service and the legal drinking age. The differences are in our guide to Korean age.

Use the age calculator

The age calculator does the arithmetic for you:

1. Enter the date of birth.
2. Leave “age on this date” as today, or set it to another date, for example a sports cutoff or a school start date.
3. Read the exact age in years, months and days, with the total in days, weeks and hours below it.

For the next birthday, a countdown shows the days left. To check a result by hand, take the birth date, add the years and months the calculator shows, and confirm you land on today.

Frequently asked questions