A year is a leap year if it divides evenly by 4, with one exception: century years, the ones ending in 00, are leap years only if they also divide by 400. So 2024 is a leap year, 2000 is a leap year, but 1900 and 2100 are not. The leap year calculator applies this rule to any year and tells you why.
That is the test in full. Here is how to run it and why the century exception is there.
The three-step test
Take the year and work through these checks in order:
- If it does not divide by 4, it is a common year. Stop.
- If it divides by 4 but is a century year ending in 00, go to step 3. Otherwise it is a leap year.
- If that century year divides by 400, it is a leap year. If not, it is a common year.
So 2023 fails at step 1, 2024 passes at step 2, and 1900 reaches step 3 and fails because it does not divide by 400.
Why the century exception exists
A solar year is about 365.2422 days, a little under 365 and a quarter. Adding a leap day every four years assumes exactly 365.25, which is slightly too long. Over centuries that small error adds up. Skipping the leap day in three century years out of four trims it back, keeping the calendar aligned with the seasons across thousands of years.
Worked examples
- 2024 divides by 4 and is not a century year, so it is a leap year.
- 1900 divides by 4 but is a century year that does not divide by 400, so it is a common year.
- 2000 divides by 4 and by 400, so it is a leap year.
- 2100 is a century year that does not divide by 400, so it is a common year.
What a leap year changes
In a leap year, February has 29 days instead of 28, and the year has 366 days rather than 365. That extra day shifts every date from 1 March onward by one day of the year. To see the day count and the previous and next leap years for any year, enter it in the leap year calculator.
For how this affects month lengths, see how many days are in each month.