The 4, 100 and 400 rule
The full leap year rule takes one sentence: a year is a leap year if it divides exactly by 4, unless it also divides by 100, in which case it must divide by 400 as well. The checker above shows each of those three tests separately so you can see exactly which one decided the answer for your year.
Most years are settled by the first test alone. 2026 does not divide by 4, so it is an ordinary 365 day year; 2024 does, and is not a century year, so it had its 29 February. The interesting cases are the century years, where the two exceptions fight it out.
Why leap years exist at all
The Earth does not orbit the Sun in a tidy number of days. One trip takes about 365.2422 days, and that leftover quarter of a day is the whole problem. If every calendar year were 365 days, the seasons would arrive about a day later every four years, and within a few centuries the calendar would say June while the weather said March.
Julius Caesar's fix in 45 BC was one extra day every fourth year, giving an average year of 365.25 days. Close, but slightly too generous: the Julian calendar runs fast by about 3 days every 400 years, which is why by the 1500s the spring equinox had drifted 10 days out of place. The Gregorian reform of 1582 removed 3 leap days per 400 years by cancelling the century years that do not divide by 400.
The century years trip everyone up
Nobody alive today has lived through a skipped leap year. The last one was 1900 and the next is 2100, and 2000 sailed through in between because it divides by 400. That accident of timing means the "every four years" shorthand has worked flawlessly for over a century, and will keep working until 29 February 2100 fails to arrive.
The 1900 case has a famous footnote: early Lotus 1-2-3 treated 1900 as a leap year by mistake, and Microsoft Excel deliberately copied the bug for compatibility. Excel still believes 29 February 1900 existed, which is why date arithmetic around January and February 1900 can be one day out in spreadsheets to this day.
Note that the Gregorian rules only apply from 1582, and in Britain only from 1752, when 11 days were skipped to catch up. For earlier years this checker applies the modern rules retrospectively (the proleptic calendar), which is the standard convention but not what people at the time would have observed.
Leap days in real life
About one person in 1,461 is born on 29 February, and in non-leap years UK law treats their birthday as falling on 1 March for age milestones such as turning 18. The extra day also nudges anything measured in exact days: a leap year has 52 weeks and 2 days rather than 52 weeks and 1 day, which is why your birthday weekday jumps by two days across a leap year's February instead of one. To see the effect on real dates, try the day of the week finder or count exact days with the days between dates calculator.
Frequently asked questions
What makes a year a leap year?
A year is a leap year if it divides exactly by 4, unless it also divides by 100, in which case it must divide by 400 as well. So 2024 is a leap year, 1900 is not, and 2000 is. The extra day is always 29 February, making the year 366 days long instead of 365.
Was 1900 a leap year?
No. It divides by 4, but it also divides by 100 and not by 400, so the century exception removes it. The year 2000 divides by 400, so it kept its leap day. This is the rule people most often get wrong, and spreadsheet software has famously got it wrong too: early Excel treats 1900 as a leap year for compatibility reasons.
Why do we have leap years at all?
The Earth takes about 365.2422 days to orbit the Sun, not a neat 365. Without correction the calendar would drift by roughly a day every four years, and after a few centuries midsummer would fall in what the calendar called spring. Adding a day roughly every fourth year keeps the calendar locked to the seasons.
Will 2100 be a leap year?
No. It divides by 4 and by 100 but not by 400, so the century rule skips it. The same applies to 2200 and 2300, while 2400 will be a leap year. Nobody alive today has experienced a skipped leap year, because 2000 happened to pass the 400 test.
When did these rules start?
The 4, 100 and 400 system is the Gregorian calendar, introduced in 1582. Before that the Julian calendar made every fourth year a leap year with no exceptions. Britain and its colonies only switched in 1752, skipping 11 days in September to catch up, so results for very old years depend on which calendar you mean.