In this guide
Working out how old someone is sounds simple until the birthday has not happened yet this year, or the months turn out to have different lengths. This guide shows the exact method, with worked examples you can check by hand.
The quick method: years first
Subtract the birth year from the current year, then check whether the birthday has already happened this year. If it has not, subtract one. Someone born on 15 March 1990 is 36 in June 2026, but on 1 March 2026 they would still be 35.
Adding months and days
Exact age is normally written as years, months and days. After the full years, count the full months since the last birthday, then the days left over. Take someone born on 28 November 1995, on 10 February 2026:
- The last birthday was 28 November 2025, so the person is 30 full years old.
- From 28 November to 28 January is 2 full months.
- From 28 January to 10 February is 13 days: 3 days left in January plus 10 in February.
The result is 30 years, 2 months and 13 days. When the day of the month is smaller than the birth day, you borrow the length of the previous month, which is what an age calculator does automatically.
Total days, weeks and hours
To count total days, use the real number of days between the two dates. A quick estimate is age in years multiplied by 365.25, because the extra quarter day covers leap years. For someone aged 36 that gives about 13,149 days. Divide by 7 for weeks and multiply by 24 for hours.
Age on any other date
The same method works for any target date, such as a school cut-off, a race entry deadline or a job requirement. Replace "today" with the date you care about and count from the birth date to that day.
Leap day birthdays
People born on 29 February have a real birthday only every four years. In other years the birthday is treated as 28 February or 1 March, depending on the place and the purpose. Our leap year guide explains the rules.
Does the time of birth matter?
For everyday and legal purposes, age changes on the birthday date in the local calendar. Counting age in exact hours depends on the time of birth and the time zone, so date-based calculators count whole days.
Age counting around the world
Most of the world counts age as 0 at birth and adds a year on each birthday. Traditional East Asian age counting starts at one and adds a year at New Year. South Korea officially moved to the international method for most legal purposes in June 2023.
Common mistakes
- Subtracting years without checking whether the birthday has passed.
- Treating every month as 30 days.
- Forgetting leap days when counting total days.
- Entering the dates in the wrong order, which gives a negative result.
Try the calculators
- Age Calculator by Date of Birth: Exact age, total days lived, next birthday
- Birthday Calculator: Day you were born, zodiac, generation
- Days Between Dates Calculator: Count days, weeks and weekdays
General information only, not professional advice. See our editorial policy.