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 "as of" date
Default is today, but you can compute age as of any past or future date — useful for ceremony eligibility, 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 — not 24 years minus 5 days, which 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 birthday only every four years; most legal systems treat 1 March as the notional birthday in non-leap years.
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 — no guesswork. The Gregorian calendar was adopted by different countries in different years, so pre-1752 dates in Britain and its colonies 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 usually treat March 1 as the notional birthday in non-leap years, so a “leapling” ages by one year every March 1 except in leap years (where it is February 29). Total days alive is calculated precisely regardless.
For Gregorian dates (most of Europe from 1582, Britain and colonies from 1752, Russia from 1918) yes. Dates before Gregorian adoption in that country follow the Julian calendar and differ by 10-13 days. Most online tools assume Gregorian 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 “as of” date. Useful for retrospective eligibility (was the person 18 on 2016-11-08?), historical records or actuarial calculations.
For years and months no — that would be unusual. Total hours yes, if you enter a birth time. Otherwise it assumes 00:00 on the birth date and the current moment on the “as of” date.