The rule: divisible by 4, with two exceptions carved out
A leap year is a year with 366 days instead of 365, the extra day being February 29. Which years qualify follows one rule with two refinements, the same rule this checker applies and prints against your year in the breakdown table: 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, in which case they are. So 2028 qualifies (divides by 4, not a century), 2000 qualifies (a century, but divides by 400), and 1900 and 2100 do not (centuries that fail the 400 test). Most years you will ever check are settled by the first clause; the exceptions exist purely because of the astronomy explained below, and they bite exactly once a century — 2100 is the next trap, with its previous leap year in 2096 and its next in 2104, an eight-year gap the checker shows if you enter it.
Why the rule exists: the 365.2422-day problem
The calendar's job is to keep dates aligned with the seasons, and the seasons are set by the tropical year — the time from one spring equinox to the next — which is about 365.2422 days, not 365. A plain 365-day calendar therefore drifts by almost a quarter of a day every year: after a century the seasons would arrive about 24 days early, and after 750 years summer and winter would have swapped months. Adding a leap day every four years overcorrects slightly — it assumes a year of 365.25 days, too long by 0.0078 days a year, which still accumulates to about three extra days every 400 years. Skipping three of every four century leap days (the 100 and 400 rules) removes almost exactly that excess, leaving the Gregorian calendar wrong by roughly one day in 3,200 years. Good enough that no further reform is scheduled, and the reason the rule has the odd shape it does.
Leap years, 2000 to 2050
The fifty years around the present follow the simple rhythm — every fourth year, with no century exception until 2100: 2000, 2004, 2008, 2012, 2016, 2020, 2024, 2028, 2032, 2036, 2040, 2044 and 2048. The table below the calculator lists the same years by decade and updates the verdict for any year you enter, past or future: the rule is pure arithmetic, so it works for 1752 or 2400 exactly as it does for next year. Practical uses cluster around February 29 itself — birthdays that only exist one year in four (celebrated on February 28 or March 1 in common years, by personal or local legal convention), contracts and subscriptions that price by the day and meet a 366-day year, and any day-count arithmetic, like the tools in this Date section, where a span crossing February behaves differently depending on which February it crosses.
Worked example
Enter 2028: 2028 ÷ 4 = 507 exactly, and 2028 is not a century year, so it is a leap year — February 2028 has 29 days and the year has 366. The leap years around it are 2024 before and 2032 after. Now try 2100: it divides by 4 and by 100 but not by 400, so it is not a leap year — the checker is one of the few places that gets 2100 right without thinking.
How to use this calculator
- Enter the year you want to check.
- Read the verdict in the green panel: leap year or common year.
- Check the breakdown table to see the 4, 100 and 400 tests applied to your year one by one.
- Note the previous and next leap years, and how many days that February has.
- Browse the 2000–2050 table below to place your year in the run.
Leap seconds, leap babies and other edge cases
A different correction, the leap second, occasionally adds a single second to atomic time to keep clocks aligned with Earth's slightly irregular rotation — it shares the name and nothing else; leap years are calendar arithmetic and are never announced or adjusted. People born on February 29, sometimes called leaplings, hold a genuine claim to the rarest birthday: the chance of being born on it is about 1 in 1,461, and jurisdictions differ on whether their legal birthday in common years falls on February 28 or March 1, which matters for coming-of-age dates. And the eight-year gap around a skipped century year (2096 to 2104) is the longest wait the modern calendar ever produces — anyone planning a very long project across 2100 should count February days with this page, not from habit.