Age Calculator
Enter a birth date and the calculator returns your exact age down to the day, plus total days and hours alive, the weekday you were born on, and a countdown to the next birthday. Handles leap years, historical dates and date ranges spanning different months correctly.
How to calculate an exact age
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1
Pick the birth date
Use the date picker or type YYYY-MM-DD. Historical dates back to 1900 (and earlier where supported) work.
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2
Choose the reference date
Default is today, but you can compute age on any past or future date. It is useful for eligibility checks, historical records or age comparisons.
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3
Read the breakdown
Output shows years, months and days, plus total days, total hours and the birth weekday.
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4
Check the next birthday
The countdown tells you how many days until the next birthday and on which weekday it falls.
How years, months and days are computed
The standard method (and the one ISO 8601 date math follows):
- Take full years between the two dates, respecting leap years.
- From the remainder, take full months using the actual length of each intervening month (28, 29, 30 or 31 days).
- The last remainder is days.
So someone born 2000-03-15, on 2024-03-10, is 23 years, 11 months and 26 days. Saying “24 years minus 5 days” would be ambiguous.
Leap year rules
A year is a leap year if:
- It is divisible by 4, AND
- If divisible by 100, it must also be divisible by 400.
That makes 2000 a leap year (divisible by 400) but 1900 and 2100 are not. A person born on 29 February has a calendar birthday only in leap years. In non-leap years, many legal and administrative systems use 1 March as the notional birthday, but local rules can vary.
Weekday of any date
The calculator uses Zeller’s congruence (or the equivalent POSIX algorithm). For any Gregorian date from 1583 onwards, the weekday is deterministic, with no guesswork. The Gregorian calendar was adopted by different countries in different years, so dates before local adoption need special handling.
Total days alive: a reality check
| Age | Total days |
|---|---|
| 10 years | ~3,652 |
| 20 years | ~7,305 |
| 30 years | ~10,957 |
| 50 years | ~18,262 |
| 70 years | ~25,567 |
| 100 years | ~36,524 |
The exact total depends on how many leap years fall in the interval. A person born 2000-01-01 has seen 6 leap years by 2024-01-01.
Frequently Asked Questions
Legal and medical systems often treat March 1 as the notional birthday in non-leap years, so someone born on February 29 is commonly treated as a year older on March 1 except in leap years. Local rules can differ. Total days alive is calculated precisely regardless.
For Gregorian dates, yes. Many Catholic countries adopted the Gregorian calendar from 1582, Britain and its colonies from 1752, and Russia from 1918. Dates before local adoption may follow the Julian calendar and differ by 10 to 13 days. Most online tools assume the Gregorian calendar throughout.
Because months have different lengths (28 to 31 days). The tool counts complete calendar months between dates, which is more accurate than a naive subtraction.
Yes. Change the reference date. Useful for retrospective eligibility checks, such as whether someone was 18 on 2016-11-08, historical records or actuarial calculations.
For years and months, no. That would be unusual. Total hours can use a birth time if you enter one. Otherwise it assumes 00:00 on the birth date and the current moment on the reference date.
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