How to use the half birthday calculator
- Enter the date of birth. Only the month and day decide the half birthday; the year is used for the age.
- Leave "Count from" on today, or pick another date to look ahead from, for example the start of a school term.
The result shows the date of the next half birthday, how many days away it is and the age reached ("36 and a half"), the month and day of the half birthday in every year, and the next five half birthdays with their day of the week. If today is the half birthday, it says so.
The rule this calculator uses
Wiktionary defines a half birthday as The day halfway in between two consecutive birthdays.
There is no official rule for finding that day, so this page uses the simplest one, and the one most people mean:
half birthday = birthday + 6 calendar months, on the same day number.
That works for most dates, but some months are shorter than others. Six months after August 31 is February, and there is no February 31. When the month has no day with that number, the calculator uses the last day of that month. This is the same rule the age calculator uses for a February 29 birthday in a common year.
The birthdays this affects:
| Birthday | Six months later would be | Half birthday used |
|---|---|---|
| August 29 | February 29 (missing in common years) | February 28, or February 29 in leap years |
| August 30 or 31 | February 30 or 31 | February 28, or February 29 in leap years |
| March 31 | September 31 | September 30 |
| May 31 | November 31 | November 30 |
| October 31 | April 31 | April 30 |
| December 31 | June 31 | June 30 |
A February 29 birthday has no such problem: six months later is August 29, which exists every year.
Half birthday for every month
For a birthday on the 15th, the half birthday is simply the 15th six months later. The last day of each month is where the rule above comes in (February is shown with its leap-year last day, the 29th):
| Birth month | Born on the 15th | Born on the last day | Half birthday |
|---|---|---|---|
| January | July 15 | January 31 | July 31 |
| February | August 15 | February 29 | August 29 |
| March | September 15 | March 31 | September 30 |
| April | October 15 | April 30 | October 30 |
| May | November 15 | May 31 | November 30 |
| June | December 15 | June 30 | December 30 |
| July | January 15 | July 31 | January 31 |
| August | February 15 | August 31 | February 28 (February 29 in leap years) |
| September | March 15 | September 30 | March 30 |
| October | April 15 | October 31 | April 30 |
| November | May 15 | November 30 | May 30 |
| December | June 15 | December 31 | June 30 |
Birthdays from July to December have their half birthday in the next calendar year: someone born on November 3 has a half birthday on May 3 of the following year.
Worked examples
All counted from October 22, 2026.
| Date of birth | Next half birthday | Days to go | Age then |
|---|---|---|---|
| May 15, 1990 | November 15, 2026 | 24 | 36 and a half |
| August 31, 2000 | February 28, 2027 | 129 | 26 and a half |
| October 22, 2026 (born today) | April 22, 2027 | 182 | half a year old |
For the August 31 birthday, the half birthdays after that run February 29, 2028 (a leap year), then February 28 in 2029, 2030 and 2031.
Six months or half a year of days?
"Halfway between two birthdays" can also be read as half the number of days. A year of 365 days has its midpoint 182.5 days after the birthday, so this method gives a date 182 or 183 days later. That can land a few days before or after the six-month date:
- Born May 15: six months later is November 15, which is 184 days after May 15, 2026. Halfway by days is November 13 or 14.
- Born August 31: six months later is February 28, 2027, which is 181 days on. Halfway by days is March 1 or 2.
Neither is official. This calculator uses the calendar month rule because it keeps the same day number every year and is easy to check on a calendar. To count the days between any two dates yourself, use the days between dates calculator.
Questions and answers
How do I work out my half birthday?
Add six months to your birthday and keep the same day number. Born March 10? Your half birthday is September 10. If the month is too short, use its last day: March 31 becomes September 30.
When is the half birthday for someone born on August 31?
February 28, or February 29 in a leap year. Six months after August 31 is February, which has no 31st, so the calculator uses the last day of February.
When is the half birthday for someone born on February 29?
August 29, every year. Only the birthday itself moves in common years; six months later is August 29, which always exists.
Is a half birthday exactly 182 days after a birthday?
Not with the six-month rule. Six calendar months is anywhere from 181 to 184 days, depending on the months in between. If you prefer half the days of the year, count 182 or 183 days on with the days between dates calculator.
Is my half birthday in the same year as my birthday?
Only if you were born from January to June. A birthday from July to December has its half birthday in the next calendar year.
How many days until my half birthday?
Enter your date of birth above and leave the count from date on today. The calculator shows the date of your next half birthday and the number of days to go, counting today as day 0.
Sources
Checked on 22 October 2026. If a rule has changed, please tell us.