Birthday Calculator

What this calculator does

Find out how many days are left until your next birthday, and what day of the week it lands on. You also see the age you'll turn and the date of your half birthday. A table shows the weekday of each of your next 10 birthdays, so you can spot the ones that fall on a weekend.

Enter your date of birth. The countdown starts from today, but you can pick any other date to count from. That's handy for planning a party, booking a trip or checking a date months ahead.

Where it's used

Why this math matters outside the classroom:

• Party planning. Parents count the 140 days from October 26, 2026 to a March 15, 2027 birthday to book a party room in time.
• Classroom celebrations. Teachers mark half birthdays for students born in summer, such as a January 15 half birthday for a July 15 birthday.
• Weekend birthdays. Friends check which of the next 10 birthdays fall on a Saturday or Sunday to plan a trip.
• Calendar practice. Students count the days in each month to find that an August 31 birthday is 309 days after October 26.

How to use it

1. Enter your date of birth.
2. Leave "Counting from" as today, or change it to count from another date.
3. Read the days left, the date and weekday of your next birthday, and the age you'll turn.
4. Check your half birthday below, then open the table to see the weekday of each of your next 10 birthdays.

What the inputs mean

Date of birth

Your calendar date of birth. The time of day isn't used. It stays on your device and is never put in a shareable link.

Counting from

The date to count from. It starts as today's date where you are. It can't be earlier than your date of birth.

Formula and method

next birthday = your birth month and day in the current year, or next year if that date has passed
days until your birthday = next birthday − counting-from date
age you'll turn = year of the next birthday − birth year
half birthday = your birthday + 6 months (moved to the month's last day if that day doesn't exist)

The calculator first checks whether your birthday is still to come this year. If it is, that's your next birthday. If it has passed, the next one is a year later. If the counting date is your birthday, the count is 0 and the page says it's today.

The day count is the number of midnights between the two dates. Each month's real length is used, and so is February 29 in leap years. To find the weekday, the days are split into whole weeks. The leftover days tell you how many weekdays to move on.

Your half birthday is the same day of the month, 6 months after your birthday. Some months are shorter than others, so that day doesn't always exist. When it doesn't, the calculator uses the last day of that month. For example, an August 31 birthday has a half birthday on February 28, or February 29 in a leap year. A March 31 birthday gives September 30.

If you were born on February 29, your birthday only exists in leap years. In other years, this calculator puts it on February 28, so you celebrate in the same month. Some people use March 1 instead. Pick whichever your family uses.

Worked example

You were born on August 31, 1995. Today is Monday, October 26, 2026. How long until your birthday, and when is your half birthday?

1. Your birthday this year, August 31, 2026, has already passed. So your next birthday is August 31, 2027.
2. Count the days: 5 left in October, then 30 + 31 + 31 + 28 + 31 + 30 + 31 + 30 + 31 days for November through July, then 31 in August. That's 309 days.
3. You'll turn 2027 − 1995 = 32.
4. 309 days is 44 whole weeks and 1 day. One weekday on from Monday is Tuesday, so your birthday is on a Tuesday.
5. For the half birthday, go 6 months on from August 31, 2026. February 2027 has no 31st, so it's February 28, 2027. That's 5 + 30 + 31 + 31 + 28 = 125 days away, on a Sunday.

Result: Your next birthday is in 309 days, on Tuesday, Aug 31, 2027, when you turn 32. Your half birthday is Sunday, Feb 28, 2027, in 125 days. You were born on a Thursday.

Understanding the result

The day count doesn't include today. If it says 1 day, your birthday is tomorrow.

The weekday table helps with planning. A birthday moves on one weekday each year, or two when a February 29 comes in between. So a Saturday birthday this year may not come around again for several years.

Some families celebrate half birthdays, especially for children with summer birthdays. Those children are often on vacation when their real birthday comes around, so the class can celebrate the half birthday instead.

Assumptions and limitations

• Dates use today's standard (Gregorian) calendar.
• A February 29 birthday is placed on February 28 in years without a February 29.
• The day you were born doesn't count as a birthday. On that day, your first birthday is a full year away.
• A half birthday that lands on a missing day moves to the last day of that month.
• The time of day isn't used, and "today" comes from your device's date. If you're counting near midnight or across time zones, the count may be one day off from a friend's.
• Some laws and organizations treat March 1 as the birthday for people born on February 29. If the date matters legally, check which rule applies.
• There's no single rule for half birthdays. A few people count exactly 182 or 183 days instead of 6 calendar months, which can give a slightly different date.

Frequently asked questions

How this calculator is checked

Method: Gregorian calendar arithmetic: day counts from a proleptic Gregorian day number; half birthday by adding 6 calendar months with end-of-month clamping; weekday from the day number. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-26. Found a problem? Tell us.

Embed this calculator

Put this Birthday Calculator on your own website or blog for free. Paste this code where you want it to appear: