Age Calculator
What this calculator does
This calculator determines your exact age in years, months, and days based on your date of birth and a reference date (today by default, or any date you choose). It can also calculate the age difference between two people, or the time elapsed between any two dates expressed in human-friendly units.
Age seems like a simple concept, but the exact calculation involves nuances—varying month lengths, leap years, and the convention of when a birthday "counts." This tool handles all of those details precisely, making it useful for legal age verification, milestone planning (retirement eligibility, insurance age bands), medical contexts (pediatric growth charts, age-adjusted health metrics), and personal curiosity.
Whether you are checking if someone meets an age requirement, figuring out how many days until a birthday, or calculating the age gap between two people, this tool gives instant, precise results.
How to use it
- Enter your date of birth (month, day, year).
- Optionally change the reference date (defaults to today).
- Review your age in years, months, and days, plus total days lived, and the next birthday countdown.
Formulas used
Years: Count full years from birth year to reference year, adjusting down by 1 if the birthday has not yet occurred in the reference year.
Months: After full years, count remaining full months from the last birthday anniversary to the reference date.
Days: After full years and months, count remaining days to the reference date.
Total days lived = Reference date − Birth date (accounting for leap years). A leap year occurs every 4 years, except centuries not divisible by 400. So 2000 was a leap year; 1900 was not.
Next birthday = The next occurrence of the birth month/day on or after the reference date. Days until = Next birthday − Reference date.
Examples
Example 1 — Standard age check: Born March 15, 1990. Today = February 28, 2026. Age = 35 years, 11 months, 13 days. Total days ≈ 13,099. Next birthday in 15 days.
Example 2 — Leap day birthday: Born February 29, 2000. Reference = March 1, 2025. Age = 25 years, 0 months, 1 day (in non-leap years, the birthday is typically recognized on March 1 or February 28 depending on jurisdiction).
Example 3 — Age difference between two people: Person A born June 10, 1985. Person B born November 22, 1988. Difference = 3 years, 5 months, 12 days.
Tips and common mistakes
- Leap year birthdays: People born on February 29 have a legal birthday every year (February 28 or March 1 depending on local law), but the calculator shows exact elapsed time regardless.
- Legal age cutoffs vary by jurisdiction. In some places you "turn 18" at the start of your birthday; in others, at the end. This calculator shows full elapsed years, which is the most common convention.
- Time zones can shift the date. If you were born just before or after midnight, the calendar date depends on the time zone. This calculator uses calendar dates only, not timestamps.
- Month-length variation means age in months is approximate. February has 28–29 days; months range from 28 to 31 days. The calculator follows the convention of counting from the birth day to the same day number in subsequent months.
- Historical dates may use different calendar systems (Julian vs. Gregorian). This calculator uses the Gregorian calendar, which is standard for modern dates.