Birthday Calculator
Enter a date of birth to get your age, days until your next birthday, the date of that birthday and the weekday you were born. Includes a worked example.
Results
Age (years)
Days until birthday
- Next birthday
- June 15, 2027
- Born on
- Friday
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Educational estimates · not financial advice · free, no signup
Formula / how it works
Age in whole years uses the last birthday. Days until next birthday is calendar-day difference.
Detailed guide
What it is, how to use it, how to read the numbers, common mistakes, a worked example, formulas, and FAQs.
What this calculator is
A birthday calculator answers four small questions at once: how old are you in completed years, how many days remain until the next birthday, which date that birthday falls on, and which day of the week you were born. You enter a date of birth and, optionally, an as-of date; leaving it blank uses today. It is handy for countdowns, party planning, forms that ask for age on a given date, and trivia. Age here is counted in completed years, the way most legal and everyday contexts do. People born on 29 February are a special case, because the birthday only exists in leap years.
When to use / who it helps
- You want a quick countdown to an upcoming birthday.
- You need to know the weekday of a birth date for a form, story or curiosity.
- You want age on a specific past or future date by setting the as-of date.
- You are planning an event and need the days remaining until a birthday.
- You want to double-check an age calculation made by hand.
How to use
Steps
- Enter the date of birth.
- Leave the as-of date empty to use today, or choose another date.
- Read the age in completed years.
- Read the days until the next birthday and the date it falls on.
- Check the weekday you were born on.
- Change the as-of date to see ages on other dates, such as a deadline.
How to interpret results
- Age counts only completed years; the day before a birthday you are still the previous age.
- Days until birthday counts calendar days from the as-of date to the next occurrence of the birth month and day.
- The next birthday date is shown with its year so you can see whether it falls next year.
- The weekday of the birth date never changes, but the weekday of each birthday shifts yearly.
- For a 29 February birth date, conventions differ on whether the non-leap-year birthday is 28 February or 1 March.
Common mistakes & gotchas
- Counting age by subtracting the birth year only, which is off by one before the birthday.
- Entering the date in the wrong day-month order.
- Forgetting the as-of date when working out an age for a deadline.
- Assuming a leap-day birthday falls on the same date every year.
- Using the result as an official proof of age, which requires documents.
Worked example
Worked example — born 15 June 1990, as of 26 October 2026
- Years from 1990 to 2026 is 36, and the 2026 birthday on 15 June has already passed, so the age is 36.
- The next birthday is 15 June 2027.
- Days from 26 October 2026 to 15 June 2027: 232 days.
- 15 June 1990 was a Friday.
Result: Age 36; 232 days until the birthday on 15 June 2027; born on a Friday.
To load the form's own demo values, tap Try example above the Calculate button.
How to calculate / formula
Age in whole years uses the last birthday. Days until next birthday is calendar-day difference.
Frequently asked questions
How is age calculated?
Count completed years from the date of birth to the as-of date. You become a year older on the birthday itself.
How do I find the day of the week I was born?
Enter your date of birth and read the Born on line.
How many days until my next birthday?
Enter your date of birth and leave the as-of date blank. The result counts calendar days from today.
When does my age go up?
On the birthday itself. The day before, you are still the previous age.
What about someone born on 29 February?
In non-leap years the birthday is commonly marked on 28 February or 1 March, depending on local law and convention.
Can I calculate age on a future date?
Yes. Set the as-of date to the future date, for example a deadline or an event.