A half birthday can mean six calendar months after a birthday or the midpoint in days between two anniversaries. Those dates can differ. Choose a method in the calculator. It shows how the result is counted.
Find your half birthday
Start with a birthday occurrence. The calculator starts with January 31, 2026 as an example.
Six calendar months later
July 31, 2026January 31, 2026 plus six calendar months.
Your selections stay in this page. They are not sent or saved by this calculator.
Six calendar months later
Use the same day number six months later. January 15 becomes July 15. October 2 becomes April 2 of the following year. The calculator counts forward from your selected birthday occurrence.
When the target month lacks that day number, we use its final day. August 31, 2026 becomes February 28, 2027. August 31, 2027 becomes February 29, 2028. This is the tool’s month-end convention. You can choose another celebration date.
Halfway between anniversaries
Count calendar days from one anniversary to the next. Divide the count by two. January 31, 2026 to January 31, 2027 spans 365 days. The midpoint is 182.5 days later, halfway through August 1, 2026. Six calendar months later is July 31, 2026.
A 366-day interval has a whole-day midpoint, 183 days after its start. January 31, 2024 to January 31, 2025 reaches that midpoint at the start of August 1, 2024.
The calculator treats each day as an equal unit. It excludes birth times, daylight-saving changes, and durations in seconds. A half-day result means the middle of that date. You can celebrate on that date or the next. No single half-birthday convention is required.
February 29 and anniversary choices
In a year that contains February 29, its six-month result is August 29. When an anniversary year lacks that date, choose February 28 or March 1. The calculator uses your choice for either missing endpoint before counting months or finding the midpoint.
February 29, 2024 to February 28, 2025 spans 365 days. Using March 1, 2025 instead gives 366 days. These choices give different midpoints. Select your celebration date. The tool does not determine legal age.
The U.S. Naval Observatory's Gregorian leap-year rule explains leap years. Years divisible by four contain February 29, except century years not divisible by 400. Thus, 2000 is a leap year and 2100 is not. The tool uses Gregorian dates from 1600 onward.
Your birthday's other numbers
A half birthday is a calendar calculation. For your month and day’s frequency in historical U.S. records, see your birthday rank. Read the data methodology. The leap-day comparison explains why calendar appearances matter for February 29's rank.
For the year when your age matches your birth day number, use the golden-birthday calculator.