How to Calculate Age from Date of Birth (Formula Explained)
The exact borrowing method calendars use — with a worked example and leap-year notes.
Age calculation looks simple until the birthday hasn't happened yet this year. Here's the exact method official documents use.
The borrowing method
- Subtract birth year from current year.
- If the birth month hasn't occurred yet this year, subtract 1 more year.
- If it's the birth month but the birth day hasn't occurred yet, subtract 1 more year.
Worked example
Born 14 March 2001, today is 11 October 2026: 2026 − 2001 = 25. March already passed and the 14th passed, so no subtraction — age is 25 years. If today were 10 March 2026, the birthday wouldn't have occurred yet: age would still be 24.
Months and days
For the exact figure (25 years, 6 months, 27 days), compute the same borrowing logic down through months and days — or just use the calculator above, which does it instantly including leap years.
Leap years
Someone born on 29 February legally ages on 28 February or 1 March in non-leap years depending on jurisdiction — the day count still works out; only the "birthday" label shifts.