What Is a Leap Year? The /4, /100, /400 Rule Explained

Why we need leap years at all

Earth takes roughly 365.2425 days to orbit the Sun, not a tidy 365. If we ignored that quarter-day, the calendar would drift about six hours every year, and after a century the seasons would slide by nearly a month. Leap years put the missing time back, one extra day at a time.

Adding one day every four years overcorrects slightly, because four quarter-days is a full day but the true figure is a touch less than a quarter. The 100-year and 400-year exceptions trim that tiny excess, keeping the long-run average almost exactly 365.2425 days per year.

The /4, /100, /400 rule step by step

Check three conditions in order. First: is the year divisible by 4? If not, it is a common year of 365 days. If yes, ask: is it also divisible by 100? If not, it is a leap year. If it is divisible by 100, ask one more thing: is it divisible by 400? If yes, leap year; if no, common year.

Worked cases: 2024 ÷ 4 = 506 with no remainder and 2024 is not divisible by 100, so it is a leap year. 1900 ÷ 4 works, but 1900 is divisible by 100 and not by 400, so it is a common year. 2000 is divisible by 400, so despite ending in 00 it is a leap year. This is why 2000 felt normal but 1900 and 2100 quietly skip February 29.

February 29 and the people born on it

The extra day is always February 29, the last day of February in a leap year. People born on that date, sometimes called leaplings, see their exact birthday only once every four years. Most mark it on February 28 or March 1 in common years; the legal choice varies by country.

Because the 400-year cycle contains exactly 97 leap years, the Gregorian calendar averages 365.2425 days per year — very close to the astronomical value. A tiny residual error means the calendar will only be off by about a day after some three thousand years, a problem comfortably left to the future.

Frequently asked questions

Is 2100 a leap year?

No. 2100 is divisible by 4 and by 100 but not by 400, so it is a common year with 365 days. The next century year that is a leap year is 2400.

How many days is a leap year?

A leap year has 366 days: the usual 365 plus February 29. That single extra day is why February has 29 days instead of its usual 28.

Why isn't every fourth year enough?

Because the orbital year is slightly under 365.25 days. Adding a day every four years overshoots a little, so we skip three leap days every four centuries — the 100- and 400-year rules — to fix the tiny surplus.

How can I check any year quickly?

Apply the three checks in order — divisible by 4, then the by-100 exception, then the by-400 override — or enter the year into a leap-year calculator, which returns yes or no and shows why.