Free Leap Year Checker with the Rule Explained

How the Gregorian Leap Year Rule Actually Works

A leap year checker tells you whether a given year has 366 days instead of 365, and the useful part is not the yes-or-no answer but the reasoning behind it. Students, developers writing date logic, genealogists working with old records, and anyone with a February 29 birthday all run into the same surprising fact: the rule is not simply "every four years". This tool takes any year, applies the full Gregorian rule, and shows each step so the answer is transparent rather than a black box, alongside a list of leap years in a range, the next and previous leap year, and a count between any two years. The rule has three tiers. A year is a leap year if it is divisible by 4, except that years divisible by 100 are not leap years, unless they are also divisible by 400. Written as a single test, a year Y is a leap year when Y is divisible by 4 and either Y is not divisible by 100 or Y is divisible by 400. Counting leap years in a span uses a closed-form shortcut: the number of leap years from year 1 to year n equals the whole number of quarters minus the whole number of centuries plus the whole number of four-hundreds, so the count between two years is that function evaluated at the end year minus its value just before the start year. Every step here is exact integer arithmetic with no rounding. Take the year 2100, which trips up almost everyone. It is divisible by 4, so the naive rule says leap. But it is also divisible by 100, which normally cancels the leap year, and it is not divisible by 400, so the cancellation stands and 2100 is a common year of 365 days. Contrast 2000: divisible by 4, divisible by 100, but also divisible by 400, so it is a leap year. The checker prints exactly this chain for whatever year you enter. Ask it how many leap years fall between 2000 and 2100 inclusive and it returns 25, computed as the century-adjusted count rather than by scanning every year one at a time. Why does the rule bother with the century exceptions at all? Because the Earth takes about 365.2422 days to orbit the Sun, not a tidy 365.25. Adding a day every four years over-corrects slightly, so the calendar would drift about three days every four centuries. Dropping three leap days per four hundred years — the skipped century years — brings the average calendar year to 365.2425 days, within roughly 27 seconds of the true tropical year. A developer validating a date-entry form uses the checker to confirm February 29 is only accepted in real leap years. A teacher uses the step display to show a class why 1900 was not a leap year. Someone born on February 29 uses the range list to see which coming years actually contain their birthday. One nuance worth stating: this checker applies the Gregorian rule to every year, including years before the calendar was introduced in 1582. That is called the proleptic Gregorian calendar and it is the right choice for modern date maths, but historical records from before a country's adoption date used the older Julian rule, which made every fourth year a leap year with no century exception. Britain and its colonies switched only in 1752, dropping eleven days to catch up, and Russia not until 1918, so a medieval or early-modern date can be off by several days if you assume the Gregorian rule applied at the time. The common mistake is to run a birth or event year from the 1600s or 1700s through a modern rule and treat the result as authoritative; if you are doing genealogy or historical research, check which calendar the source document used before trusting a leap-year answer. Everything runs locally in your browser; nothing you type is uploaded or stored anywhere.

Leap Year Checker Formula & Method

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