In the Gregorian calendar, a leap year is a year with 366 days, including February 29. Normally it occurs every 4 years, except if the year is divisible by 100, then it’s not a leap year—unless it’s also divisible by 400, in which case it is a leap year again. This calculator helps you easily determine if a specific year is a leap year.

Leap years correct the gap between the ~365.24-day orbital period of Earth and the 365-day calendar. The rules are as follows:

• Years divisible by 4 are leap years.
• Years divisible by 100 are not leap years.
• Years divisible by 400 are leap years again.

Example: 2020 is divisible by 4, so it’s a leap year; 1900 is divisible by 100 but not 400, so it’s a common year; 2000 is divisible by 400, so it’s a leap year.

The leap year concept dates back to ancient Rome, but the current Gregorian leap year rules were introduced in 1582 by Pope Gregory XIII. Previously, the Julian calendar simply added a leap year every 4 years, but the Gregorian reform added the 100-year and 400-year rules to reduce inaccuracies.