Date & Time
Birthday Calculator
How long until your next birthday, the day of the week you were born, and what day your next six birthdays land on.
Birthday Calculator
Hijri dates here use the tabular (arithmetical) Islamic calendar, which is fixed and predictable. Locally observed dates depend on moon sighting and can differ by a day either way — check with your local authority for religious observances.
About the Birthday Calculator
Enter your date of birth and this calculator counts down to your next birthday — in days and weeks — tells you which weekday you were born on, and shows what day of the week each of your next six birthdays falls on.
That last part is the genuinely useful bit. Knowing your birthday lands on a Saturday in two years' time is worth having well before anyone starts planning. The list runs six years ahead so you can see the pattern rather than one isolated date.
Leap-day birthdays are handled explicitly rather than being allowed to roll silently into 1 March. If you were born on 29 February, the page says exactly what it did and why. The Hijri date of your birth is shown alongside.
How to use the Birthday Calculator
- Pick your date of birth.
- Select Count down to my birthday.
- The headline is the days remaining; if it is today, it says so.
- The table shows the weekday for each of your next six birthdays.
- 29 February birthdays are handled explicitly rather than silently rolling.
Counting down to your birthday
The calculator takes your birth month and day, applies them to the current year, and checks whether that date has already passed. If it has, it moves to next year. Then it counts the actual calendar days between today and that date.
Next birthday = this year's date if still ahead, otherwise next year's
Days away = exact calendar days from today to that date
Worked example
Born 15 March 1990, checking on 6 September 2026.
15 March 2026 has already passed, so the next birthday is 15 March 2027.
From 6 September 2026 to 15 March 2027 is 190 days, or about 27 weeks.
190 days away — turning 37 — falls on a Monday
Why your birthday moves weekday each year
A common year is 365 days, which is 52 weeks plus one day. That extra day pushes your birthday forward one weekday every year. A leap year is 366 days — 52 weeks plus two — so it pushes forward two.
| Year | 15 March falls on | Shift |
|---|---|---|
| 2025 | Saturday | — |
| 2026 | Sunday | +1 |
| 2027 | Monday | +1 |
| 2028 | Wednesday | +2 (leap year) |
| 2029 | Thursday | +1 |
| 2030 | Friday | +1 |
The 2028 jump happens because 29 February 2028 falls between the 2027 and 2028 birthdays. The overall effect is that your birthday returns to the same weekday roughly every 5, 6 or 11 years rather than on a neat cycle.
Birthdays on 29 February
A leap day arrives only in years divisible by 4, skipping century years unless they divide by 400. So 2000 was a leap year, 1900 was not, and 2100 will not be. Someone born on 29 February gets a calendar-exact birthday roughly once every four years.
In the other three years, people split into two camps. Some celebrate on 28 February — staying inside the birth month. Others celebrate on 1 March — keeping the day-after-28-February position. Both are entirely reasonable, and the calculator uses 28 February, saying so on the page when it applies.
Legal systems have their own answers and they are not consistent. Some jurisdictions treat a leap-day birthday as falling on 28 February for age-of-majority purposes, others on 1 March. If a legal threshold turns on it — a licence, a pension date, a contract — check the rule that governs your case rather than assuming.
Are some birthdays more common than others
Yes, and by more than you would expect from chance. Birth records across several countries show clear seasonal patterns:
- Late summer and early autumn peak in many northern-hemisphere countries — mid-July through late September is consistently busiest.
- Late December and 1 January dip sharply, partly because scheduled deliveries avoid public holidays.
- 29 February is by far the rarest, appearing on the calendar only once in four years.
- Weekends see fewer births than weekdays in countries with high rates of scheduled delivery.
Planning around the countdown
Once you know the weekday, planning gets easier. A midweek birthday usually means celebrating the weekend before or after; a Friday or Saturday needs booking further ahead. The six-year list is there so you can spot a milestone birthday landing on a weekend and plan properly.
For your exact age rather than the countdown, use the age calculator. For your birthday on the Islamic calendar, try the Hijri age calculator.
Half birthday calculator
Your half birthday is six months after your birthday. Kids love it. So do people born near Christmas, who want a party that isn't buried in the holidays.
To find it, add six months to your birth date. Keep the same day number. Born on 14 March? Your half birthday is 14 September.
| Birthday | Half birthday |
|---|---|
| 5 January | 5 July |
| 14 March | 14 September |
| 20 June | 20 December |
| 25 December | 25 June |
| 31 August | 28 February (29 in a leap year) |
The last row is the tricky one. February has no 31st, so the half birthday moves back to the last day of February. You can check any date with the date calculator — add 6 months to your birth date.
Countdown until my birthday
The result above counts the days to your next birthday and shows the weekday it falls on. It also lists your next six birthdays, so you can spot the next one on a Saturday. Handy for booking a venue early.
The countdown always uses whole days and treats today as day zero. So if your birthday is tomorrow, the countdown reads 1 day.
Birthday paradox calculator
How many people need to be in a room before two of them probably share a birthday? Most people guess around 180. The real answer is just 23.
It sounds wrong, but the maths is solid. With 23 people there are 253 possible pairs, and each pair is a chance for a match. The odds climb fast:
| People in the room | Chance two share a birthday |
|---|---|
| 10 | 11.7% |
| 23 | 50.7% |
| 30 | 70.6% |
| 50 | 97.0% |
| 70 | 99.9% |
P(no match) = (365 × 364 × … × (365 − n + 1)) ÷ 365n
P(at least one match) = 1 − P(no match)
This is the birthday problem formula. It assumes 365 equally likely birthdays and ignores 29 February, which barely changes the result.
Hebrew (Jewish), lunar and Hijri birthday calculator
Some calendars follow the moon, so your birthday in them moves around the Gregorian year.
- Hijri (Islamic) birthday — the result above shows your Hijri birth date. A Hijri year is about 11 days shorter, so this birthday arrives roughly 11 days earlier each year. The Hijri age calculator gives your next one.
- Hebrew (Jewish) birthday — the Hebrew calendar is lunisolar. It adds a leap month seven times in every 19 years, so your Hebrew birthday drifts within about a month of your normal one, then swings back. This calculator does not convert Hebrew dates; a dedicated Hebrew calendar converter will.
Other ways to use this calculator
Birthday maths comes up in more ways than a countdown.
- Leap year birthday calculator — enter a 29 February birth date to see which day your birthday falls on in non-leap years.
- Lunar and Hijri birthday calculator — the result shows your Hijri birth date; your next Hijri birthday is on the Hijri age calculator.
- Half birthday calculator — add six months to your birth date — the table above lists common dates.
- Next birthday calculator and birthday countdown — see the days, hours and weekday of your next birthday.
- What day was I born? — the result shows the weekday of your birth date.
- Hebrew birthday calculator — the Hebrew date of a birthday needs a Hebrew calendar converter; this tool counts Gregorian birthdays.
Frequently asked questions
The calculator applies your birth month and day to the current year, moves to next year if that date has passed, then counts the exact calendar days between today and it. Born on 15 March and checking on 6 September 2026, your next birthday is 15 March 2027 — 190 days, or about 27 weeks, away. It also shows the weekday it will fall on.
It is worked out from the date itself, since the Gregorian calendar repeats on a fixed 400-year cycle and every date maps to exactly one weekday. Enter your date of birth above and the tool returns it directly. The same calculation gives the weekday for each of your next six birthdays, which is genuinely useful when planning ahead.
Practice splits two ways. Some celebrate on 28 February, keeping the birthday inside the birth month; others use 1 March, keeping the day-after-28-February position. Both are perfectly reasonable and this calculator uses 28 February, saying so when it applies. Legal systems differ too, so if a licence, pension or contract date turns on it, check the specific rule.
Because 365 days is 52 weeks plus one day, so your birthday shifts forward one weekday each year. Leap years are 366 days — 52 weeks plus two — so they push it forward two. That is why a birthday might run Saturday, Sunday, Monday, then jump to Wednesday. It returns to the same weekday roughly every 5, 6 or 11 years.
Add six months to your birth date and keep the same day of the month. Born on 10 April? Your half birthday is 10 October. If that day does not exist, such as 31 August plus six months, use the last day of the shorter month.
Only 23 people give a better than even chance (50.7%) that two share a birthday. With 50 people the chance rises to about 97%. This surprising result is called the birthday paradox.
Sources and further reading
- Royal Museums Greenwich — The leap year explained — the leap year rule and 29 February birthdays
- Office for National Statistics — How popular is your birthday? — seasonal distribution of birth dates
- Gregorian calendar — Encyclopaedia Britannica — the 400-year cycle behind weekday calculation
- Hebcal — Hebrew date converter — for converting a birthday to the Hebrew calendar