"Leap years happen every 4 years" is the version most people learn as children, but it's actually incomplete — there's a lesser-known exception for century years, and then an exception to that exception, which together make the real rule more precise (and more interesting) than the simplified version.
The leap year system exists to keep the calendar aligned with Earth's actual orbital period, which is approximately 365.2422 days — not a clean 365. Adding a leap day every 4 years accounts for most of that extra quarter-day, but it slightly overcorrects, which is exactly why the century exception exists to fine-tune the alignment further.
The Complete Leap Year Rule
A year is a leap year if it's divisible by 4, EXCEPT century years (divisible by 100), UNLESS the year is also divisible by 400.
In formula form: Leap Year = (Year % 4 == 0 AND Year % 100 != 0) OR (Year % 400 == 0)
A Worked Example
2024 is divisible by 4 and not a century year, so it's a leap year — straightforward under the basic rule. 1900 is divisible by 4, but it's also a century year (divisible by 100) and NOT divisible by 400, so it was NOT a leap year — despite fitting the simple "every 4 years" rule. 2000, by contrast, is divisible by 100 (a century year) but also divisible by 400, so it WAS a leap year — the exception-to-the-exception kicking in.
Common Mistakes to Avoid
- Applying only the "every 4 years" rule without checking the century exception: this correctly identifies most leap years but gets century years wrong roughly 3 times out of every 4 centuries.
- Assuming all century years follow the same pattern: 1900 was not a leap year, but 2000 was — the divisible-by-400 exception is what makes the difference.
- Forgetting February 29th only exists in leap years: date calculations spanning a leap year's February need to correctly account for the extra day.
- Confusing the reason for leap years with the rule itself: leap years exist to keep the calendar aligned with Earth's orbit — the century exception exists specifically because a clean 4-year cycle very slightly overcorrects for that misalignment.
Bottom Line
The full leap year rule accounts for century-year exceptions that "every 4 years" alone misses. Use a Leap Year Checker to instantly verify whether any given year qualifies under the complete rule.