How age is calculated in years, months and days
Working out age in whole years is easy. Working it out in years, months and days is trickier, because months have different lengths. This guide explains the method and its edge cases.
Open the Age CalculatorWhole years
Age in years is the number of birthdays that have passed. Subtract the birth year from the current year, then subtract one if this year’s birthday has not happened yet.
Example: born 14 March 1995. On 1 October 2026 the 2026 birthday has passed, so the person is 2026 − 1995 = 31. On 1 February 2026 it had not, so the person was 30.
Adding months and days
For the remaining time, count whole months from the last birthday, then the days from the last completed month. Using the same example on 1 October 2026:
- 14 March 2026 to 14 September 2026 is 6 whole months.
- 14 September to 1 October is 17 days.
The answer is 31 years, 6 months and 17 days.
The tricky part is that a “month” is not a fixed number of days, so the day count depends on which months you passed through. That is why different calculators sometimes disagree by a day or two.
Birth dates at the end of a month
What is one month after 31 January? There is no 31 February. SizeKit, like many calculators, treats the last day of the shorter month as the anniversary, so one month after 31 January is 28 February (29 in a leap year). Counting this way keeps the result predictable and avoids negative day counts.
Leap day birthdays
People born on 29 February have a calendar birthday only in leap years. In other years, the calendar either skips the date or moves it to 28 February or 1 March. Different legal systems treat the legal birthday differently, so the calculator’s answer should be confirmed for any legal purpose.
Checking an age cut-off
Forms, schools and sports leagues often ask for age “as of” a particular date. Enter that date in the optional field of the age calculator to see exactly how old someone is on it. Also check whether the rule says “must be at least” or “must be under”, because that changes which side of the birthday counts.
Some places and traditions count age differently. For example, traditional East Asian age reckoning counted a baby as one year old at birth, and South Korea moved to the international way of counting in 2023. If a form is from a different system, read its definition of age.