How do I calculate my half birthday?

Add six months to your birth month and keep the same day. Born March 15? Your half birthday is September 15. If the resulting date doesn't exist (like February 31), it moves to the last day of that month.

What is the half birthday of someone born August 31?

Six months after August 31 would be February 31, which doesn't exist. The half birthday becomes February 28 — or February 29 in a leap year.

Why do people celebrate half birthdays?

Kids with birthdays in summer break or near big holidays often celebrate their half birthday at school instead. For adults it's a lighthearted mid-year excuse for cake, and for babies the six-month mark is a real developmental milestone.

Half Birthday Calculator

Your half birthday is the date exactly six months from your birthday — a second, smaller celebration hiding in your calendar. Enter your birth date to find it, plus a countdown to the next one.

Find your half birthday

Your date of birth

Your half birthday

Next half birthday
Falls on a
Days until then

What is a half birthday?

A half birthday is the date that falls exactly six months after your birthday — the midpoint between one birthday and the next. Born on April 10? Your half birthday is October 10. It's the same simple rule for everyone, with one tricky exception covered below.

Who actually celebrates these?

Kids with summer birthdays. A child born in July never gets to bring cupcakes to class. Celebrating the half birthday in January fixes that — it's the most common reason schools and parents use the date.
Babies at six months. The six-month mark is a genuine developmental milestone, and many parents mark it with a photo shoot or a "half cake."
People with holiday birthdays. If your birthday is December 25 or January 1, a June celebration means your day finally gets its own spotlight.
Anyone who likes an excuse for cake. Half a cake, technically.

The February 31 problem

Six months after August 31 would be February 31 — a date that doesn't exist. The standard rule (and the one this calculator uses) is to roll back to the last real day of the month: February 28, or February 29 in a leap year. The same fix applies to any 31st that lands in a 30-day month.

Frequently asked questions

How do I calculate a half birthday by hand?

Add 6 to your birth month and keep the day. If the month goes past December, wrap around to the next year. If the day doesn't exist in the target month, use that month's last day.

Is a half birthday exactly 182.5 days?

Not usually. Because months have different lengths, the six-calendar-month convention lands anywhere from 181 to 184 days after your birthday. The calendar-month rule is the one nearly everyone uses.

Can I find a half birthday for any year?

Yes — your half birthday falls on the same month and day every year (with the February exception shifting between the 28th and 29th).

Is my birth date stored?

No. Everything runs in your browser; nothing is sent to a server.