Birthday Calculator
Result
Days until the next birthday
- Weeks until the next birthday
- 12.9 weeks
- Next birthday
- January 1, 2027
- Weekday of the next birthday
- Friday
- Age reached on that day
- 37
- Weekday of the birth date
- Monday
A birthday calculator counts the days until the next birthday, tells you which day of the week it lands on, and gives the age you will be turning. It reads today's date itself, so there is nothing to enter but the date of birth. The weekday figure is the one people actually come for: it is what you need in order to book a party, a restaurant or a day off. Because a birthday is a fixed month and day, the countdown shrinks by one each day and resets on the morning after the birthday itself. It also answers the other direction — what day was I born on — from the same date of birth.
Formula
next birthday = the birth month and day placed in this year if that date has not passed, otherwise in next year; days until = next birthday minus today; age turning = the year of the next birthday minus the year of birth
- birthDate
- The date of birth. A future date is allowed — a due date counts down the same way
- daysUntilBirthday
- Whole days from today to the next birthday; 0 when today is the birthday, never 365
- weeksUntilBirthday
- The same countdown in weeks, to one decimal place
- nextBirthday
- The date the countdown lands on, which is this year's birthday or next year's
- nextBirthdayWeekday
- The day of the week that date falls on; the figure to check before booking anything
- ageTurning
- The age reached on that date: the year it falls in minus the year of birth
- birthWeekday
- The day of the week the person was born on; a weekday number, not a date
Use it to plan something that happens on a birthday rather than to state how old someone is: booking a table, ordering a cake, arranging leave, or working out whether the party should be on the Saturday before or after. It is also the right tool when the date is still in the future — a due date, a first birthday that has not happened yet, or a milestone someone is counting towards, all of which the age calculator refuses. Reach for the age calculator instead when the question is how old someone is today, and for the anniversary calculator when the thing being counted is years since an event rather than a birthday.
Worked examples
Born 1 January 1990, read on 15 January 2026
- This year's birthday would be 1 January 2026, which is already 14 days in the past
- So the countdown moves to 1 January 2027
- 15 January 2026 to 1 January 2027 is 351 days (365 minus the 14 already gone)
- 351 divided by seven is 50.14, which rounds to 50.1 weeks
- 1 January 2027 is a Friday, and 2027 minus 1990 is 37, so that is the age being turned
The birthday that has just passed makes the countdown look long — 351 days for someone who turned 36 a fortnight ago. That is correct: the page counts towards the next one, and the age calculator is where the age you already are lives.
A leap-day birthday: 29 February 2000
- The birth month and day are 29 February, and 2026 is not a leap year
- The day is clamped to the last day of the month, so the next birthday is 28 February 2026
- 15 January 2026 to 28 February 2026 is 44 days
- 44 divided by seven is 6.29, which rounds to 6.3 weeks
- 28 February 2026 is a Saturday, and 2026 minus 2000 is 26
The 28th is a choice rather than a fact of nature: some places treat 1 March as the birthday in a common year, and which one applies depends on the jurisdiction. This page clamps, and the FAQ says so. The person was born on a Tuesday.
Limitations
Today is taken as the UTC calendar day rather than your local one, so in time zones ahead of UTC the countdown can be a day out during the first hours after midnight. A 29 February birthday is treated as 28 February in common years; some legal systems use 1 March instead, and the page does not offer that option. There is no way to ask for the countdown to a date other than today, and no way to ask about a birthday that has already passed this year. Weekday names are rendered by the browser, so they follow the page's language rather than the language of the date of birth.
Frequently asked questions
- How many days until my birthday?
- Enter the date of birth and the top row gives the answer in days, with the same countdown in weeks underneath. The count is towards the next occurrence: if the birthday has already passed this year, it counts to next year's. On the birthday itself the figure is 0 rather than 365, and it becomes 364 the following day.
- What day of the week is my next birthday?
- The weekday row names it outright, and the date row above it gives the day you are counting to. Both are needed together, because the weekday is what decides whether the party is that evening or the Saturday next to it. The countdown in days does not tell you the weekday on its own — the calendar does.
- What day was I born on?
- The birth weekday row answers that from the date of birth alone, with no counting towards a future date involved. For someone born on 1 January 1990 it reads Monday. This is the same weekday figure a day of the week calculator produces for that date, so the two pages agree by construction rather than by coincidence.
- When is a 29 February birthday in a common year?
- This page uses 28 February, clamping the day to the last day of the month. Some countries legislate 1 March instead, and a few treat the birthday as arriving on both days. Nothing in the arithmetic picks one for you: the year has no 29 February, so a choice is being made, and knowing which choice this page makes is the point of asking.
- Can I use it for a due date that has not arrived yet?
- Yes, and that is a deliberate difference from the age calculator, which rejects a future date. A due date is a birthday nobody has had yet, and the countdown to it is the whole question. The age turning reads 0, meaning the age reached on that day is birth itself. Use the age calculator once the date is in the past.