Birthday Calculator
Your birthday is more than a single number. From your date of birth, this calculator works out your exact age in completed years, the total number of days you have lived, the day of the week on which you were born, and how many days remain until your next birthday. Every figure comes straight from calendar arithmetic, so leap years and varying month lengths are handled correctly. Enter your date of birth and the results update instantly, entirely in your browser, with nothing sent anywhere.
Birthday calculation method
Total days lived = whole days from birth date to today
Age in years = year difference, minus 1 if this year's birthday has not occurred
Next birthday = birth month and day in the current or next year
Days until next birthday = whole days from today to next birthday
Day of week = weekday of the birth date in the Gregorian calendar
Day counts are computed from the calendar so leap years and month lengths are exact. Age increases by one only after the birthday in the current year has passed.
Calendar context
- The Gregorian calendar, introduced in 1582, is the civil calendar used in the United States and most of the world.
- A common year has 365 days; a leap year has 366, occurring when the year is divisible by 4 (except centuries not divisible by 400).
- Someone born on 29 February has a true birthday only in leap years; common years mark it on 1 March by convention.
- The day of the week repeats on a 400-year cycle in the Gregorian calendar.
- All arithmetic runs locally in your browser; no date of birth is transmitted or stored.
Birthday calculator: frequently asked questions
How does the birthday calculator find my age?
It compares your date of birth with today's date. The age in years is the difference in calendar years, reduced by one if your birthday has not yet occurred this year. The total days lived is the exact count of days between the two dates, computed from the calendar.
How are days until my next birthday calculated?
The calculator finds the next occurrence of your birth month and day on or after today. If your birthday has already passed this year it uses next year's date, then counts the whole days from today to that date. On your birthday itself the count is zero.
Does it handle leap years correctly?
Yes. The day count uses the browser's calendar arithmetic, which accounts for leap years and the differing lengths of months. People born on 29 February see their next birthday fall on 1 March in common years, following the usual convention, since 29 February does not exist that year.
What does the day of the week output mean?
It is the day of the week on which you were born, derived from your date of birth using the Gregorian calendar. The same arithmetic powers the proleptic Gregorian calendar used by date libraries, so the result matches historical records for dates after the calendar's adoption.
Is my date of birth sent anywhere?
No. The calculation runs entirely in your browser. Your date of birth is never transmitted to a server or stored. Our analytics records only that a calculation happened, never the values you typed, in line with our privacy practices.
Official sources
- U.S. Naval Observatory: Astronomical Applications, calendar data.
- NIST: Time and Frequency Division.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.