What is a leap year?
A leap year is a year with 366 days instead of 365. The extra day is 29 February, and it helps keep calendar dates aligned with the Earth's orbit around the sun.
A leap year is a year that has 366 days instead of 365. The extra calendar day is 29 February, which is why leap years affect age, birthday and date-difference calculations.
Most years have 365 days. A leap year adds one extra day because the Earth takes a little longer than 365 days to orbit the sun. Without leap years, calendar dates would slowly drift away from the seasons.
How leap years are worked out
In the Gregorian calendar, the simple rule is that a year is usually a leap year if it is divisible by 4. Century years are the exception: they are only leap years if they are divisible by 400.
Leap year rule:
If the year is not divisible by 4 → not a leap year
If the year is divisible by 4 but not by 100 → leap year
If the year is divisible by 100 but not by 400 → not a leap year
If the year is divisible by 400 → leap yearThat is why 2024 and 2028 are leap years, 1900 was not a leap year, and 2000 was a leap year.
Why leap years matter
Leap years matter whenever a calculation crosses 29 February. They can change the number of days between two dates, the day of the week a birthday falls on, and how age is displayed in years, months and days.
Check an exact age
Use the age calculator to work out exact age, total days lived and the next birthday countdown.
Leap year examples
| Year | Leap year? | Reason |
|---|---|---|
| 2024 | Yes | Divisible by 4 and not a century year. |
| 2026 | No | Not divisible by 4. |
| 1900 | No | Divisible by 100 but not by 400. |
| 2000 | Yes | Divisible by 400. |
Leap years and 29 February birthdays
People born on 29 February only see their exact calendar birthday in leap years. In non-leap years, different systems may treat their birthday as 28 February or 1 March depending on the rule being used.
A birthday or age calculator should explain how it handles 29 February, because legal, school, workplace or service rules may not all use the same convention.
For informal age and birthday calculations, the best approach is to show a clear leap-day note rather than hide the assumption.
Leap years in date counting
A leap year can add one extra day to a date range if the range includes 29 February. This is why a days-between-dates calculation may not match a rough “years × 365” estimate.
Example:
1 January 2024 to 1 January 2025 = 366 calendar days
1 January 2025 to 1 January 2026 = 365 calendar daysFor exact date calculations, count real calendar dates rather than assuming every year has the same number of days.
Leap year FAQs
How often is a leap year?
A leap year usually happens every four years, but century years are only leap years if they are divisible by 400.
Why does February have 29 days in a leap year?
The extra day is added to February to keep the calendar aligned with the Earth's orbit around the sun.
Was 2000 a leap year?
Yes. Although 2000 was a century year, it was divisible by 400, so it was a leap year.
Is 2100 a leap year?
No. 2100 is divisible by 100 but not by 400, so it is not a leap year.
Notes
- Calculatorz uses the Gregorian calendar leap-year rule for general date, age and birthday calculations.
- For legal, employment, school or official eligibility rules involving 29 February birthdays, check the wording of the specific rule being applied.