Leap Year Rules Explained

The Rule

A year is a leap year if it divides evenly by 4, unless it also divides evenly by 100, in which case it must divide evenly by 400 to qualify. In short: divisible by 4, not by 100, or divisible by 400.

Why the Exceptions Exist

A tropical year is about 365.2422 days rather than 365.25. Adding a day every four years overcorrects by roughly three days every 400 years. The 100 and 400 exceptions claw those three days back over each four century cycle.

Why 1900 Was Not a Leap Year

1900 divides by 4 but also by 100 and not by 400, so it is a common year with 365 days. This surprises people who assume the four year rule is absolute, and it is why 29 February 1900 never existed.

Why 2000 Was a Leap Year

2000 divides by 400 exactly, so it qualified as a leap year with 366 days. 29 February 2000 was a real date, which is why anyone born on that day has a legitimately rare birthday.

The Century Pattern

Within any 400 year cycle there are exactly 97 leap years. The most common non leap century year is 1900, and the next will be 2100. Checking the century rule is what distinguishes a correct calendar calculation from a plausible but wrong one.

How Far Off Could It Drift

With the full rule, the Gregorian calendar averages 365.2425 days per year against a tropical year of about 365.2422, a discrepancy of roughly 26 seconds a year. That accumulates to a full day in roughly 3,300 years.