How to calculate age from a birth date
Count finished years, then finished months, then days, and do not stop at a simple year subtraction.
Short answer
Subtract the birth year from the as-of year, then give a year back if the birthday has not occurred yet. Do the same for months and days.
Borrow a month or a year when you must
Compare the month and day with the as-of date before you trust the year gap. If today is before the birthday, the person is one year younger than the raw subtraction. If the day of the month has not arrived, the current month is not a full month either.
Leap-day birthdays only exist on February 29. A calculator should reject February 30 and should not invent a birthday. For a form that says “age attained on the anniversary,” follow that form instead of a general calendar age.
Common mistakes
- Reporting the age you will be later this year.
Frequently asked questions
- What if I need days between two dates instead?
- Use the days-between-dates tool. Age includes years and months; a span of days does not.
Related
- Age calculatorFind an age in years, months, and days from a birth date and an optional as-of date.
- How to count days between two datesCount calendar days between two dates without losing or double-counting the day a month changes.
- Days between datesCount the days between two calendar dates, including neither extra day beyond the span you picked.