"Leap years happen every 4 years" is the version most people learn, and it's right about 97% of the time — but it's not the actual rule, and the exceptions explain some genuinely odd-looking history, like why the year 1900 didn't get a February 29th.
The Actual Rule (It's More Than "Every 4 Years")
The real Gregorian calendar rule has three parts: a year is a leap year if it's divisible by 4, unless it's also divisible by 100, in which case it's only a leap year if it's also divisible by 400. Run through a few examples and the pattern clicks: 2024 is divisible by 4 but not 100, so it's a leap year. 1900 is divisible by 4 and by 100, but not by 400, so it isn't. 2000 is divisible by 4, by 100, and by 400, so it is. Three numbers divisible by 4, three different outcomes.
Why the Calendar Needs Correcting at All
The whole system exists because Earth's orbit around the sun doesn't take a tidy round number of days — it takes about 365.2422 days. A calendar that just used 365 days every year would drift out of sync with the seasons by about a quarter-day annually, which adds up to a full month of drift in roughly 130 years. Adding a leap day close to every 4 years keeps the calendar's average length near 365.25 days, which is much closer to reality.
The Century Exceptions
Even 365.25 days isn't perfectly accurate — the real figure, 365.2422, is a little less than 365.25. Adding a leap day every single 4 years without exception would very slowly overcorrect, drifting the calendar the other direction over centuries. The 100-year and 400-year exceptions trim out 3 leap days every 400 years to compensate, bringing the calendar's long-run average length even closer to Earth's actual orbital period. That's the entire reason 1900 and 2100 lose their leap day while 2000 and 2400 keep theirs.
Why February Specifically
The leap day always lands on February 29th because February is the shortest month in the modern calendar, historically the last month of the Roman calendar year before a reform moved the year's start to January. Rather than restructuring every month's length, the extra day was simply tacked onto the month that already had the fewest days — turning a 28-day February into a 29-day one in leap years.
Checking Any Year Instantly
Enter any year to see instantly whether it's a leap year, or list every leap year across a range, with our free Leap Year Checker.
FAQ
What is the exact rule for a leap year? A year is a leap year if it's divisible by 4, unless it's also divisible by 100 — in which case it's only a leap year if it's also divisible by 400. This is why 2000 was a leap year but 1900 and 2100 are not, even though all three are divisible by 4.
Why do we need leap years at all? Earth's orbit around the sun takes about 365.2422 days, not exactly 365. Adding a leap day roughly every 4 years keeps the calendar synced with the seasons; the extra 100/400-year exceptions correct for that 365.2422 figure being slightly less than 365.25, so leap years aren't added quite as often as a simple "every 4 years" rule would suggest.
How many days are in February during a leap year? 29 days instead of the usual 28 — that extra day, February 29th, is the leap day itself.
Will 2100 be a leap year? No. 2100 is divisible by 4 and by 100, but not by 400, so it fails the century exception and will not be a leap year — the next skipped century year after 1900. The next century year that will be a leap year is 2400, since that one is divisible by 400.