A half birthday calculator finds the date you turn exactly half a year older: six months after your birthday. Enter a date of birth and it shows your next half birthday, the weekday, your age as a number and a half, and the following five half birthdays, with leap years and month-end birthdays handled.
Quick answer: Half birthday = birthday + six months. A birthday on May 15 has its half birthday on November 15, and the age that day is 36½ if the person was born in 1990. The exact length of six months is 181 to 184 days, depending on the months.
How to use the half birthday calculator
- Pick your date of birth, or the birth date of a child, partner or friend.
- The second date starts as today. Change it to look for the half birthday after any other day.
- Choose how to count six months: six calendar months later, or halfway between birthdays in days.
- Optional: type a whole age, such as 59, to get the date you turn 59½.
- Read the date, the weekday, the days left and the table of the next five half birthdays.
The half birthday calculator runs in your browser and keeps the dates on your device.
What is a half birthday?
Wikipedia defines a half-birthday as a day approximately six months before or after the anniversary of a person's birth. It adds that it is sometimes marked by people whose birthday falls on or near a major holiday, which can overshadow the birthday, and by students whose birthday falls outside the school year, so they can celebrate with friends at school. It is a custom, not an official date.
How to calculate half birthday: two methods
There are two ways to work out a half birthday. The calendar method is quick: take the month of the birthday, add six and keep the day. The days method is closer to a true half: add half the days in the year to the birthday. Wikipedia notes that the easy method gives a date that is not exactly half a year away more than 75% of the time.
- Calendar method: May 15 → November 15; January 3 → July 3. Where the day does not exist, use the last day of that month.
- Days method: half of the days between two birthdays, 182.5 in a common year and 183 in a leap year. The calculator counts whole days, so 365 becomes 182 and 366 becomes 183.
- Age: the number of whole years completed plus ½, so a person who turned 36 in May is 36½ in November.
For most birthdays the two methods differ by one to three days. Use the calendar method for a party or a card, because it keeps the same day of the month, which is what people expect. The half birthday calculator uses it by default.
When is my half birthday? A lookup by birth month
This table shows the half birthday for a birthday on the 15th of each month, and how many days there are between the two dates. The gap is not always 182 days, because months have 28 to 31 days. The right column shows the gap when a February 29 falls inside the six months. The half birthday calculator gives the same result for any day of the month.
| Birthday | Half birthday | Days apart (common year) | Days apart with Feb 29 inside |
|---|---|---|---|
| January 15 | July 15 | 181 | 182 |
| February 15 | August 15 | 181 | 182 |
| March 15 | September 15 | 184 | same |
| April 15 | October 15 | 183 | same |
| May 15 | November 15 | 184 | same |
| June 15 | December 15 | 183 | same |
| July 15 | January 15 | 184 | same |
| August 15 | February 15 | 184 | same |
| September 15 | March 15 | 181 | 182 |
| October 15 | April 15 | 182 | 183 |
| November 15 | May 15 | 181 | 182 |
| December 15 | June 15 | 182 | 183 |
The shortest gap is 181 days, from January 15 to July 15, and the longest is 184, from May 15 to November 15. The second half of the year is longer than the first half, so half birthdays for spring and summer birthdays are a day or two further away.
Half birthday leap year and month-end birthdays
Not every month has a day 29, 30 or 31, so a calendar half birthday needs a rule when the day does not exist. The rule used in the half birthday calculator is the last day of the month. Wikipedia gives August 30 as an example, since six months later would be February 30, which does not exist. The table shows the half birthday of the main edge cases for both a common year and a leap year.
| Birthday | Calendar method, common year | Calendar method, leap year | Days method (common year) |
|---|---|---|---|
| August 29 | Feb 28 (common year) | Feb 29 (leap year) | Feb 27 |
| August 30 | Feb 28 (common year) | Feb 29 (leap year) | Feb 28 |
| August 31 | Feb 28 (common year) | Feb 29 (leap year) | Mar 1 |
| January 31 | Jul 31 (common year) | Jul 31 (leap year) | Aug 1 |
| March 31 | Sep 30 (common year) | Sep 30 (leap year) | Sep 29 |
| May 31 | Nov 30 (common year) | Nov 30 (leap year) | Nov 29 |
| December 31 | Jun 30 (common year) | Jun 30 (leap year) | Jul 1 |
| February 29 | August 29 | August 29 | Aug 29 (from Feb 28) |
For a leap-day birthday (February 29), the calendar method gives August 29 in every year. In common years the birthday itself is usually counted on February 28 or March 1, and the half birthday does not depend on that choice.
What day of the week is my half birthday?
A half birthday is 181 to 184 days after the birthday. Because 182 days is exactly 26 weeks, the half birthday falls on the same weekday as the birthday only when the gap is 182 days. Otherwise it shifts by the number of days over a whole number of weeks, and the half birthday calculator shows the weekday for you.
| Days between the birthday and half birthday | Weeks and days | Weekday compared with the birthday |
|---|---|---|
| 181 | 25 weeks and 6 days | One weekday earlier |
| 182 | 26 weeks | The same weekday |
| 183 | 26 weeks and 1 day | One weekday later |
| 184 | 26 weeks and 2 days | Two weekdays later |
Half birthday and ages that count half years
Some rules use a half year of age. The best known is 59½. The IRS explains that amounts taken out of an IRA or retirement plan before reaching age 59½ are generally early distributions, and an additional 10% tax applies unless an exception covers it. Wikipedia adds that the federal government defines the half year as six calendar months after the anniversary of birth, whatever day of the month that produces.
Wikipedia also lists driving permits. It says that in many US states the minimum age for a learner permit falls on a half birthday, such as 14½ in Idaho, 15½ in California, 14 years and eight months in Michigan and 15 years and nine months in Maryland. State rules change, so check the current rule with the motor vehicle agency. The Optional age box in the half birthday calculator gives the date you reach any whole age and a half.
| Date of birth | 59th birthday | Turns 59½ |
|---|---|---|
| Aug 31, 1966 | Aug 31, 2025 | Feb 28, 2026 |
| Sep 1, 1966 | Sep 1, 2025 | Mar 1, 2026 |
| Dec 31, 1966 | Dec 31, 2025 | Jun 30, 2026 |
| Jan 15, 1967 | Jan 15, 2026 | Jul 15, 2026 |
| Jun 30, 1967 | Jun 30, 2026 | Dec 30, 2026 |
Half birthday for babies: months and half birthdays
For a baby, the half birthday is the six-month mark. Parents often count a baby's age in months, so the calendar method is the natural fit. Enter the baby's date of birth in the half birthday calculator, or use this table of month marks for a baby born on March 10, 2026. The 6-month mark is the half birthday.
| Age in months | Date |
|---|---|
| 1 | Apr 10, 2026 |
| 2 | May 10, 2026 |
| 3 | Jun 10, 2026 |
| 6 (half birthday) | Sep 10, 2026 |
| 9 | Dec 10, 2026 |
| 12 | Mar 10, 2027 |
| 18 | Sep 10, 2027 |
For a baby born on the 29th, 30th or 31st, some months have no matching day. A mark on the last day of the month keeps the count in whole months.
Half birthday ideas for kids, adults and babies
Once the half birthday calculator gives you the date, here are simple ways to mark it.
- Kids: a half cake, half the candles or a half-sized party. It works well for a child with a summer birthday, since the school classmates are around in the half-year. Wikipedia mentions this custom of celebrating at school with half a cake.
- Holiday birthdays: if the birthday is on or near a major holiday, a half birthday gives it a day of its own, as Wikipedia notes.
- Adults: a dinner, a day off or a small goal check-in. Six months is a natural point to look back at a yearly goal and set the next one.
- Babies: a photo at six months, a growth note or a first-tastes day, with a simple photo in the same chair or outfit every month.
- Couples and friends: a half anniversary uses the same math. Add six months to the date.
Half birthday date and the next birthday
Between two birthdays there is exactly one half birthday. If your birthday has not passed this year, the half birthday that comes next is the one after last year's birthday, six months before this year's. The half birthday calculator shows the birthday it follows and the next birthday after it, so the sequence is clear.
Frequently asked questions
10 quick answers
What is a half birthday?
A half birthday is the day six months after your birthday, the point where you are half a year older. It is an informal custom used to celebrate children, babies and people with holiday birthdays.
When is my half birthday?
Add six months to your birthday and keep the day. For a birthday on May 15, the half birthday is November 15. The half birthday calculator does this for any date and shows the weekday.
How do you calculate half birthday?
Add six calendar months to the date of birth. If that month has no matching day, use its last day. For a more exact half, add half the days between two birthdays, 182 or 183.
Is a half birthday 182 days after your birthday?
Not always. Six calendar months is 181 to 184 days. Half of 365 is 182.5 days, so the two methods can differ by one to three days.
What is my half birthday if I was born on August 31?
By calendar months it is February 28 in a common year and February 29 in a leap year, since February has no 31st. By exact days, it is about March 1.
What is the half birthday of a leap day baby?
Six months after February 29 is August 29. The half birthday is the same every year, and the leap day only affects when the birthday itself is counted.
How old am I on my half birthday?
Your age in whole years plus one half. Someone who turned 36 in May is 36½ in November.
Why is the half birthday different from half of 365 days?
Months have different lengths, so six months is not always the same number of days. The second half of the year has more days than the first half.
Is 59½ a half birthday?
Yes. It is the half birthday after the 59th birthday. The IRS uses age 59½ for early retirement distributions, and it counts six calendar months after the anniversary of birth.
Can I use the half birthday calculator for a baby?
Yes. Enter the baby's date of birth. The half birthday is the six-month mark, and the table shows the next five.
Related calculators
- Age calculator for exact age in years, months and days
- Days until calculator to count down to a date
- Date calculator to add or subtract months and days
- Days between dates calculator for the exact gap
- All free calculators
Sources and further reading
- Half-birthday (Wikipedia): the definition, the two calculation methods and permit ages
- IRS: Retirement topics, tax on early distributions: the age 59½ rule
- February 29 (Wikipedia): leap day birthdays
This page is general information and not tax or legal advice. Last reviewed: October 2026.