Half Birthday Calculator
Find your half birthday, the date exactly six months after your birthday, plus the days left until it and your half birthday twins.
Your half birthday appears here.
Enter your birthday and choose Find my half birthday.
How your half birthday is found
We add six calendar months to your birthday and keep the same day of the month. A March 21 birthday gives a September 21 half birthday, and a December 5 birthday gives June 5.
If the day does not exist six months later, we use the last day of that month. A birthday on August 31 has a half birthday on February 28, or February 29 in a leap year.
Some people count 182.5 days, half of 365, instead. That splits the year more evenly but lands a day or two before the calendar answer, so we use calendar months, which is what most people mean. For a countdown to the full birthday, see Days Until My Birthday. You can also find your golden birthday or everyone who shares your birthday with the Birthday Twin Finder.
Worked example
Born March 21. Six calendar months later is September 21, so that is your half birthday every year. Counting 182 days instead lands on September 19, and 183 days on September 20, which is why some calculators give a different answer.
Half birthdays for month-end birthdays
Most half birthdays keep the same day of the month. These are the exceptions.
| Birthday | Half birthday |
|---|---|
| March 31 | September 30 |
| May 31 | November 30 |
| August 29, 30 or 31 | February 28 (February 29 in leap years) |
| October 31 | April 30 |
| December 31 | June 30 |
Half Birthday Calculator questions
What is my half birthday?
Your half birthday is six months after your birthday. If you were born on March 21, your half birthday is September 21. Enter your birthday above to see yours and how many days are left.
What is the half birthday for August 31?
February 28, or February 29 in a leap year. February has no 31st, so the half birthday moves to the last day of the month. The same rule gives August 29 and August 30 birthdays a February 28 half birthday in common years.
How do you find your half birthday?
Add six months to your birthday and keep the same day of the month. If that month is too short, use its last day. Some people count 182 or 183 days instead, which can land a day or two earlier.
What are half birthday twins?
People whose birthday falls on your half birthday. If you were born on March 21, everyone born on September 21 is your half birthday twin: your birthday is their half birthday, and theirs is yours.
Do people celebrate half birthdays?
Some do, especially children born near the holidays or in summer, when a class party on the real birthday is hard to arrange. A half birthday gives them a celebration in the school year.
Last checked October 2, 2026. Uses the Gregorian calendar and your device's time zone.