Leap Year Checker

Leap year checker with the rule explained

Is 2028 a leap year? Was 1900? Will 2100 be? This leap year checker answers instantly for any year and shows each step of the Gregorian rule, so you can see why. It also compares the result with the older Julian calendar, gives the previous and next leap year, tells you which weekday the next 29 February falls on, and lists every leap year in any range you choose.

It is handy for programmers testing date code, students learning the rule, planners working out February deadlines, and anyone born on 29 February wondering when their next “real” birthday is.

How to use the Leap Year Checker

1. Type a year into the “Year” box, or use “Previous” and “Next” to step through the years.
2. Read the verdict and the number of days in the year and in February.
3. Follow the three checks — divisible by 4, by 100 and by 400 — to see how the answer was reached.
4. Check the extra details: the Julian calendar’s answer, the previous and next leap years and the weekday of the next 29 February.
5. To list leap years, enter a “From year” and “To year”. Copy the list with one tap.

Leap Year Checker features

The leap year rule

In the Gregorian calendar used almost everywhere today, a year is a leap year if it is divisible by 4, except years divisible by 100, unless they are also divisible by 400. So 2024 and 2028 are leap years; 1900 and 2100 are not, because they are century years not divisible by 400; and 2000 and 2400 are leap years because they are divisible by 400.

In code the rule is one line: (y % 4 === 0 && y % 100 !== 0) || y % 400 === 0. Testing only y % 4 === 0 is a classic bug that gives the wrong answer for 1900 and 2100. Even Microsoft Excel treats 1900 as a leap year — a quirk kept for compatibility with Lotus 1-2-3 — so dates in early 1900 can be off by one day in spreadsheets.

Why leap years exist

Earth takes about 365.2422 days to go once round the Sun from one March equinox to the next — the tropical year. A calendar of exactly 365 days would slip almost a quarter of a day every year, and after a century the seasons would arrive about 24 days early.

Julius Caesar’s Julian calendar, which took effect in 45 BC, fixed most of this by adding a day every fourth year, giving an average year of 365.25 days. That is still about 11 minutes too long, which adds up to roughly one day every 128 years. By the 1500s the spring equinox had drifted ten days, so in 1582 Pope Gregory XIII dropped ten days from the calendar and introduced the century rule. Skipping three leap days every 400 years gives 97 leap years per 400 years and an average year of 365.2425 days — close enough that the calendar drifts by only about one day in more than 3,000 years.

The extra day sits in February because the early Roman year ended with February. The Romans repeated the sixth day before the start of March, which is where the old word for a leap year, “bissextile”, comes from.

Leap year facts

• Upcoming leap years: 2028, 2032, 2036, 2040, 2044 and 2048. The next century year that is not a leap year is 2100.
• The chance of being born on 29 February is about 1 in 1,461. Many “leaplings” celebrate on 28 February or 1 March in common years; use the Age Calculator to count your birthdays exactly.
• A leap year has 366 days and 52 weeks plus 2 days, so dates after February shift two weekdays forward from the year before instead of one. The Day of the Week Calculator shows this for any date.
• Leap seconds are something different: they are occasionally added to clock time to keep it in step with Earth’s irregular rotation, not with the orbit.
• Need a printed February with all 29 days? The Calendar Generator handles leap years automatically.