How to calculate age from a date of birth
An age calculator compares a birth date with a selected date. Unlike a simple years-subtraction formula, it accounts for whether the birthday has occurred and for the lengths of individual months.
Step-by-step method
- Select your date of birth.
- Select today’s date or a different date for historical or future age.
- Read the number of completed years, completed months after the last birthday, and remaining days.
- Review total days lived and the next birthday countdown.
Age in years, months and days
The calculator determines the most recent birthday on or before the calculation date. Then it counts complete calendar months since that anniversary, followed by the remaining days. Date arithmetic uses the Gregorian calendar.
Age calculation example
A person born on January 1, 2000 is 25 years, 0 months and 0 days old on January 1, 2025. On January 2, 2025, the age becomes 25 years, 0 months and 1 day.
How do leap years affect age?
Leap years include February 29. For a date of birth on February 29, this calculator uses February 28 as the birthday anniversary in years without a February 29. This is a stated calculation convention rather than a claim about legal age determination.
Next birthday countdown
The calculator reports the next birthday date, how many calendar days remain, and the age you will turn. On a birthday, the countdown is zero.
Important notes
Age is calculated from the calendar dates entered, not from a live clock time. Legal, administrative and eligibility rules may apply different conventions, especially for leap-day birthdays and age cutoffs. Verify such requirements with the relevant authority.