Basic age calculation
Start by subtracting the birth year from the current year. Then determine whether the birthday has occurred yet in the current year.
Adjust for the birthday
If the person's birthday has not yet occurred in the current year, subtract one from the initial year difference.
Example
Someone born on December 20, 2000 is not yet 26 on October 1, 2026. The year difference is 26, but because the December birthday has not occurred, the completed age is 25.
Age in years, months and days
A detailed age calculation compares calendar dates rather than assuming every month has the same number of days. Month lengths and leap years can affect the remaining months and days.
Why leap years matter
Leap years contain February 29, so date calculations spanning February can contain one additional calendar day compared with a typical year.
Common uses for age calculations
- Checking exact age on a particular date
- Eligibility calculations
- School or event age requirements
- Finding elapsed time since birth
- Comparing ages between dates
Exact age should be based on calendar dates rather than assuming a fixed number of days in every month or year.