What Is a Leap Year? The Rule, the Reason, and the Next Ones

Every four years February grows a 29th day — except when it doesn't. The leap year rule has a famous exception that trips up even programmers. Here's the whole story, simply.

Why we need leap years

Earth takes about 365.2422 days to orbit the Sun, not a tidy 365. Those extra ~6 hours a year add up; without correction, the calendar would drift and eventually you'd be celebrating New Year in summer. Adding a day every four years keeps the calendar aligned with the seasons.

The rule (with the famous exception)

A year is a leap year if it's divisible by 4 — except century years, which must also be divisible by 400. So 2024 ✓, 2025 ✗, 1900 ✗ (divisible by 100 but not 400), 2000 ✓. Check any year instantly with the leap year checker.

Why the 100/400 twist exists

Adding a day every 4 years slightly over-corrects (it assumes exactly 365.25 days). Skipping the leap day on most century years removes the excess; adding it back on every 400th year fine-tunes it. The result keeps the calendar accurate to within one day over ~3,000 years.

The next leap years

2028, 2032, 2036 and so on — every fourth year through this century, since none of them are century years. The next skipped one is 2100, which will not be a leap year despite being divisible by 4.

Born on February 29?

"Leaplings" technically have a birthday once every four years and usually celebrate on Feb 28 or Mar 1 in common years. Curious how many actual days you've been alive across all those leap years? The age calculator counts your exact age in years, months and days.