Check whether any year is a leap year, see why, and find the nearest previous and next leap years instantly.
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This tool checks whether any year you enter is a leap year, following the standard Gregorian calendar rule: a year is a leap year if it's divisible by 4, except that century years (like 1900 or 2100) are not leap years unless they're also divisible by 400 (which is why 2000 was a leap year but 1900 and 2100 are not). Beyond a simple yes-or-no answer, it explains exactly which part of the rule applies to your chosen year, shows how many days that year and its February have, and reports the nearest leap years both before and after it. This is useful for anyone planning an event around February 29, writing software that needs to handle calendar edge cases correctly, or simply settling a factual question about a specific year.
Most people know the rough idea that leap years happen "every four years," and for the vast majority of years, that shorthand is correct. But it's not the whole rule, and the exception to it is exactly the kind of detail that trips people up when they reason about specific years by hand — especially century years like 1900, 2000, and 2100, which don't all follow the simple every-four-years pattern the way 2024 or 2028 do.
The reason leap years exist at all comes down to astronomy, not arbitrary tradition. Earth takes approximately 365.2422 days to complete one orbit around the sun — not a clean 365. If calendars simply used 365 days every year, the calendar would drift out of sync with the seasons by about a quarter of a day annually, adding up to a full month of drift roughly every 131 years. Julius Caesar's calendar reform in 46 BCE addressed this by adding a leap day every four years, which gets much closer to the true solar year but still slightly overcorrects, since 365.25 days per year (the effective average under a strict every-four-years rule) is a bit more than the actual 365.2422.
Pope Gregory XIII's calendar reform in 1582 refined this further with the rule still in use today: a year is a leap year if it's divisible by 4, except that century years (years divisible by 100) are not leap years, unless they're also divisible by 400. Under this rule, 2000 was a leap year (divisible by 400), but 1900 was not (divisible by 100 but not 400), and neither will 2100 be. This adjustment brings the average Gregorian calendar year to 365.2425 days, remarkably close to the true 365.2422-day solar year — close enough that the Gregorian calendar only drifts by about one day roughly every 3,300 years.
Under this system, leap years occur 97 times every 400 years, rather than the 100 times you'd get from a naive every-four-years rule. That means if you're counting leap years across a long span of history or projecting far into the future, you can't just divide the number of years by four — you also need to account for the century-year exception and its own exception. This calculator handles that arithmetic directly, so you don't have to work through the divisibility rules by hand or risk miscounting around a century boundary.
Leap years matter beyond pure calendar trivia. Software systems that handle dates need to account for February having 29 days in leap years, and bugs related to mishandling this edge case are common enough to have their own name in programming circles ("leap year bugs"). People born on February 29 — sometimes called "leaplings" — only have a true calendar birthday once every four years on average, and often celebrate on February 28 or March 1 in other years. Financial calculations, payroll systems, and scheduling software all need to handle the extra day correctly to avoid off-by-one errors. This tool is meant as a quick, reliable reference for any of these situations: enter a year, and see immediately whether it's a leap year, why, and which leap years are nearest to it in either direction.
A leap year is a calendar year with an extra day, February 29, added to keep the calendar year synchronized with Earth's orbit around the sun. Leap years have 366 days instead of the usual 365.
A year is a leap year if it is divisible by 4, except that century years (divisible by 100) are not leap years unless they are also divisible by 400. This is why 2000 was a leap year but 1900 and 2100 are not.
A year is about 365.2422 days long, slightly less than 365.25. Adding a leap day every 4 years alone would overcorrect, so the Gregorian calendar drops the leap day in century years not divisible by 400, bringing the average calendar year much closer to the true length of a solar year.
Leap years occur every 4 years in most cases, giving roughly 97 leap years every 400 years under the Gregorian calendar rule, rather than the 100 you would get from a simple every-4-years rule.
This tool applies the standard Gregorian leap year rule proleptically (extended backward) to any year you enter, which matches how modern software and most calendar calculations handle historical dates, even though the Gregorian calendar itself was only adopted starting in 1582.