Guides/Age from date of birth
How to calculate age from date of birth
Exact age is more than “this year minus birth year.” You also need months and days since the last birthday, relative to an as-of date (often today).
Step-by-step
- Subtract birth year from as-of year for a provisional year count.
- Compare month and day: if the birthday has not occurred yet this as-of year, subtract one year.
- For remaining months/days, subtract birth month/day from as-of month/day, borrowing days from the previous month when needed.
- Optionally count total calendar days lived (end − start in whole days).
Worked examples
- Born 1998-05-10, as of 2026-05-10: exactly 28 years, 0 months, 0 days.
- Born 1998-05-10, as of 2026-08-25: 28 years, 3 months, 15 days.
- Born 1998-05-10, as of 2026-03-01: birthday not reached → 27 years plus months/days from 2025-05-10.
- Leap day births: Feb 29 anniversaries vary by jurisdiction and form; calendar tools borrow days in non-leap years.
Common mistakes
- Using year subtraction alone (off by one before the birthday).
- Dividing total days by 365 and calling it exact age.
- Confusing age with a generic date difference in weeks —weeks between dates.
Calculate instantly
Use the freeAge Calculatorfor years / months / days and an optional as-of date. For total days/weeks framing, seeexact age in days, weeks, monthsor theDate Difference Calculator.