Leap Year Checker

Is 2100 a leap year? (No — and that surprises people.) Enter any year to find out instantly, with a plain-English explanation of which rule decided it, plus the next and previous leap years. You can also list every leap year between two years.

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How to use the Leap Year Checker

Type a year and the verdict appears immediately: whether it is a leap year, the specific reason (which of the divisibility rules applied), and the closest leap years before and after it. The default, 2024, is a leap year; try 1900 or 2100 to see the century exception in action.

To see leap years across a span, fill in the range fields and click List. The tool shows every leap year from the start to the end year inclusive, along with a count — useful for planning, scheduling, or just satisfying curiosity about how often they fall. The years are listed in order so you can scan the four-year rhythm and spot where a century year breaks it.

Everything is computed locally with simple integer arithmetic, so it is instant and works for any year, past or future.

The leap year rule explained

A leap year has 366 days instead of 365, adding 29 February to keep the calendar aligned with Earth's orbit. The reason we need them is that a year is not a whole number of days: Earth takes about 365.2422 days to circle the Sun. If we ignored that extra quarter-day, the calendar would drift against the seasons by roughly a day every four years, and within a few centuries summer and winter would land in the wrong months.

The rule that keeps things aligned has three parts, applied in order. A year is a leap year if it is divisible by 4 — that handles the quarter-day by adding a day every four years. But four quarter-days slightly overcorrects, so the second rule removes the leap year when a year is divisible by 100 (the century years like 1900 and 2100). That removes a touch too much, so the third rule adds it back when a year is divisible by 400 (so 2000 is a leap year, but 1900 and 2100 are not). The net effect is 97 leap years every 400 years, which tracks the true orbital period to within about one day in 3,000 years.

This is the Gregorian calendar, introduced in 1582 to fix drift that had accumulated under the older Julian calendar, which used only the divisible-by-4 rule and so had slightly too many leap years. The century exceptions are the part people forget — most living programmers have only seen one century year, 2000, and it happened to be a leap year, which quietly hides the rule. That is exactly why date libraries are tested against 1900 and 2100, and why "just check divisible by 4" is a classic bug.

Common use cases

  • Verifying date logic. Confirm your code's leap-year handling against known tricky years like 1900, 2000, and 2100.
  • Planning around February 29. Find when the next leap day falls for scheduling, billing, or birthdays.
  • Teaching the rule. Show students the divisible-by-4/100/400 logic with live examples and a range list.
  • Quick answers. Settle "is this year a leap year?" without doing the arithmetic in your head.

Frequently asked questions

Why is 2100 not a leap year?
It is divisible by 100 but not by 400, so the century exception applies and it is a common year. 2000 was divisible by 400, which is why it was a leap year and 2100 is not.

What is the simple version of the rule?
Divisible by 4 is a leap year, except divisible by 100 is not, except divisible by 400 is. Checking only "divisible by 4" is wrong for century years.

How often is there a leap year?
Almost every four years, but three times every 400 years a century year is skipped. That gives 97 leap years per 400-year cycle rather than 100.

Does this work for years before 1582?
It applies the Gregorian rule to any year you enter. Historically, dates before the Gregorian reform used the Julian calendar, so for those the proleptic Gregorian result shown here may differ from the calendar in use at the time.