Age Calculator
Enter a birth date and see the calendar age in years, months, and days, or the days until the next birthday. The third option is a share of a fixed 100-year span. It is not a life expectancy.
Smart Tips
• Age is calendar subtraction of years, months, and days from the birth date to the as-of date.
• Leave the as-of date empty to use today. The count uses calendar dates, not a time zone.
• Share of 100 Years is days lived divided by 100 × 365.25, capped at 100%. It is not a life expectancy.
• A February 29 birth is valid only in a leap year. In a non-leap year the next birthday falls on March 1.
Formulas
| Type | Formula |
|---|---|
| Exact Age | Years, months, and days by calendar subtraction. If the day has not been reached, borrow the previous month’s length. If the month has not been reached, borrow 12 months. |
| Days Lived | Whole days from birth midnight to the as-of midnight. |
| Share of 100 Years | Percent = days lived ÷ (100 × 365.25) × 100, capped at 100%. Years until 100 = max(100 − completed calendar years, 0). The percent and the year count use different bases. |
Key Insights
Leap Years & Month Lengths
Dates follow the Gregorian calendar: a leap year every 4 years, century years that are not leap years, and years divisible by 400 that are. February 29 is accepted only in a leap year. In a non-leap year the next-birthday date falls on March 1.
Share of 100 Years
The percent uses days lived, not completed years divided by 100. Years until 100 subtracts completed calendar years from 100 and stops at 0. Neither number is a life expectancy.
As-of date
An as-of date on or after the birth date shows the calendar age on that date, including a later date. The calculator does not apply a time zone.
Next Birthday
Days until the birth month and day in the as-of year, or the next year if that date has passed. February 29 rolls to March 1 when that year is not a leap year.
Frequently Asked Questions
Note:Calendar age only. The bar is a share of a fixed 100-year span (days lived ÷ (100 × 365.25), capped at 100%), not a life expectancy. Years until 100 is 100 minus completed calendar years.