Leap Year Calculator

Check if a year is a leap year, find the next and previous leap years, and see how many leap years are in the century.

A year qualifies as a leap year under the Gregorian rule this calculator applies (line 16) when it's divisible by 4 -- unless it's also divisible by 100, in which case it only still counts if it's divisible by 400 too. That third clause is what makes most people's mental shortcut wrong: 1900 is divisible by 4 and by 100 but not by 400, so it was not a leap year, while 2000 cleared all three tests and was. Days in Year follows directly from that single check -- 366 when it passes, 365 when it doesn't (line 18). Next Leap Year and Previous Leap Year are found by stepping one year at a time, forward or backward from the year you entered, until the rule passes again (lines 20-24); Previous Leap Year additionally stops at year zero rather than searching indefinitely into negative years. Leap Years in Century counts every qualifying year across the run of 100 years starting at your entered year's own century floor -- entering 2024 counts leap years across 2000-2099, not just "this century" in the ordinary sense of 2001-2100. A century that itself contains its own divisible-by-400 year, like 2000-2099, holds 25 leap years; an ordinary century that doesn't, like 1900-1999, holds only 24, because the century-boundary year loses its leap status under the divisible-by-100-but-not-400 exception. This calculator only implements the Gregorian rule -- it doesn't account for the Julian calendar or any pre-1582 calendar reform, so results for historical years before the Gregorian switch describe what the rule would say, not what calendar was actually in use at the time.