How to Calculate Age from Date of Birth
How to calculate age from date of birth in years, months, and days. Manual steps, worked examples, leap-year notes — plus a free age calculator.
How to calculate age from date of birth
Exact age = completed years + leftover months + leftover days from date of birth to the as-on (end) date. Use the age calculator to do it instantly, or follow the manual method below when you need to show your working.
how to calculate age
What exact age means when you calculate age
Exact age on forms is almost never a decimal like 27.4 years. It is a calendar breakdown:
- Years — full birthdays reached
- Months — full months after that last birthday anniversary toward the end date
- Days — leftover days after those months
Total days lived is a separate number. Useful for curiosity; rarely what a legal form wants.
age calculator
Manual method (Y/M/D)
- Write the end date and the birth date as Year / Month / Day.
- Subtract days. If end days are smaller, borrow the number of days in the previous calendar month of the end date.
- Subtract months. If needed, borrow 12 months from the year (and reduce years by 1).
- Subtract years.
Worked example
End date: 8 Nov 2026
Birth date: 20 Feb 1999
- Days: 8 < 20 → borrow from October (31 days) → days = 8 + 31 − 20 = 19; months reduce by 1
- Months: (11 − 1) − 2 = 8
- Years: 2026 − 1999 = 27
Result: 27 years, 8 months, 19 days.
Second example (birthday not reached)
End: 10 March 2026 · Birth: 25 March 2000
- Days: 10 < 25 → borrow from February 2026 (28 days) → days = 10 + 28 − 25 = 13; months borrow 1
- Months: (3 − 1) − 3 → need another borrow from years → months = (3 − 1 + 12) − 3 = 11; years reduce by 1
- Years: 2026 − 2000 − 1 = 25
Result: 25 years, 11 months, 13 days.
Shortcuts that go wrong
- Dividing total days by 365 (or 365.25) and treating the integer as completed years
- Subtracting years only and ignoring whether the birthday has passed
- Assuming every month has 30 days when borrowing
- Rounding 11 months up to the next year
More: common age calculation mistakes.
Leap-day births
People born on 29 February still have an exact age every year. In non-leap years there is no 29 Feb; everyday observance is commonly treated as 28 February. Exam notices may define their own rule — read the text. Detail: leap-year birthday age.
How to calculate age today vs as on a date
Same method; different end date. For jobs and exams, the end date is the printed as-on / closing date. Walkthrough: calculate age as on a date.
FAQ
How do I calculate exact age from date of birth?
Subtract calendar days, then months, then years from the end date to the birth date, borrowing across month lengths when needed. Or use the free age calculator for the same completed-age result.
Is exact age the same as total days ÷ 365?
No. Exact form age is years, months, and days on the calendar. Day totals answer a different question.
What end date should I use?
Today for current age; the printed as-on / closing date for jobs and exams.