How to calculate your age

The quick method gives the year; the borrow method gives years, months and days. This guide covers both, plus the leap-year and birthday traps and why your age differs between cultures.

By the Calculator Den team · Updated 2026-11-04

Your age is the number of complete years since the day you were born. There are two ways to work it out: a quick method for the year and an exact method for years, months and days. Both share the same trap, whether your birthday has happened yet this year. This guide shows the fast way, the exact borrow method, how to handle leap years and end-of-month birthdays, how to find your age on a future or past date, and why your age is not even the same number in every culture. The worked example below was checked against the age calculator.

The quick way to calculate your age in years

Subtract your birth year from the current year, then subtract one more if your birthday has not happened yet this year. That is the whole method.

Age = current year − birth year − (1 if your birthday is still to come this year, else 0)

Example: born in 1998, and today is in 2026. 2026 − 1998 = 28. If your birthday has already passed this year, you are 28. If it is still to come, you are 27. That adjustment is the easiest step to miss, and skipping it makes a bare year-minus-year subtraction one year too high whenever the birthday is still to come.

How to calculate your exact age in years, months and days

For an exact age, treat it like long subtraction of two dates written as year, month, day, with today on top and your birth date underneath, and borrow when a column goes negative. Work right to left: days, then months, then years.

Worked example: born 20 July 1990, and today is 4 November 2026. Write it as 2026-11-04 over 1990-07-20.

• Days: 4 − 20 is negative, so borrow one month. Add the days in the previous month (October has 31) to the 4: 4 + 31 = 35, then 35 − 20 = 15 days. The month count drops by one, from 11 to 10.
• Months: 10 − 7 = 3 months.
• Years: 2026 − 1990 = 36 years.

So the age is 36 years, 3 months and 15 days. If the months had gone negative too, you would borrow a year the same way: add 12 to the months and reduce the years by one.

Leap years and the 29 February birthday

The borrow method handles leap years automatically, as long as you use the real number of days in the month you borrow from (February is 29 in a leap year, 28 otherwise). Leap years matter most for a 29 February birthday, known as a leapling. In common years with no 29 February, practice varies: some places and calculators use 28 February and others use 1 March for the purpose of turning a year older, and legal rules on this differ by country. A child born on 29 February 2000, for example, turns 26 on 28 February or 1 March 2026, depending on the rule used. Over long spans, remember that a year averages about 365.2425 days in the Gregorian calendar (365.25 is the common rule of thumb), which is why age in total days is not years times 365.

How to calculate your age on any date, and the difference between two ages

The same method finds your age on any date, not just today: put the target date on top instead of today and subtract your birth date as before. For the 20 July 1990 birth date, age on 1 January 2030 works out to 39 years, 5 months and 12 days (days: 1 − 20 borrows December's 31, giving 12; then borrow a year so months are 12 − 7 = 5; years 2029 − 1990 = 39). That answers how old you will be at a future event, or how old you were on a past one. To find the age difference between two people, subtract one birth date from the other with the same borrow method, which gives the gap in years, months and days.

If you only need the gap in total days, for example days until your next birthday, that is a date-difference rather than an age: count the days between today and the date. From 4 November 2026, the next 20 July birthday is 20 July 2027, which is 258 days away. Our guide to days between two dates covers that counting, including the inclusive-versus-exclusive trap, and a quick way to check any age is to add the result back to your birth date and confirm you land on today.

Why your age differs by culture

The age you have been calculating is the international standard: you are zero at birth and turn a year older on each birthday. But it is not universal. In the traditional East Asian age system, still common informally in parts of the region, a person is counted as one at birth and gains a year at the new year (lunar in some places, 1 January in others) rather than on their birthday, which can make someone up to two years older than their international age. South Korea standardised on international age for most legal and administrative purposes in June 2023. So if two sources give different ages for the same person, a counting convention, not an error, is often the reason. This article uses the international method throughout.

Common mistakes when calculating your age

• Forgetting the birthday adjustment. Year minus year is one too high if this year's birthday has not happened yet. Subtract one more in that case.
• Not borrowing correctly. When days go negative, add the number of days in the previous month, not a fixed 30. If that still leaves the days negative (born on the 31st, say), you have hit an end-of-month edge case, covered below.
• Using 365 for total days. Over several years a year averages 365.2425 days, so multiplying age in years by 365 undercounts the days.
• Mixing up age with days between dates. Age is completed years (plus months and days); days until a birthday is a plain date-difference count.

Edge case: birthdays on the 29th, 30th or 31st

The borrow method can run into a snag when your birth day is near the end of a long month. Born on 31 January, with today 1 March: borrowing one month adds February's days (28 in a common year), giving 1 + 28 = 29, which is still less than 31. There is no clean answer, so different systems resolve it differently: some count it as the last day of the shorter month, others roll into the next month. A calculator and a hand count can then differ by a day or two around these dates. It only affects people born on the 29th, 30th or 31st, and only in the few days around a shorter following month, but it is why two tools can disagree slightly on the same birthday.

For the year, subtract birth year from this year and take one off if the birthday is still to come; for exact age, use the borrow method on the two dates. When you want it instant, the free age calculator returns your exact age in years, months and days from your date of birth, and the date difference calculator counts the days between any two dates, such as the days until your next birthday.