How to use it
- Type a year (the current year is filled in). The answer and the rule check update as you type.
- Read the three tests: divisible by 4, a century year, divisible by 400. The step that decides is highlighted.
- Use the previous / next buttons to jump between leap years, including across century boundaries.
- Enter a range below to list every leap year in it and count them.
Example
2100: divisible by 4 (2100 ÷ 4 = 525) – but it is a century year, and 2100 ÷ 400 = 5.25, so it is not a leap year. The previous leap year is 2096 and the next one 2104, an eight-year gap. Between 2000 and 2100 inclusive there are 25 leap years; between 1900 and 2000 there are also 25, because 1900 is skipped but 2000 counts.
The Gregorian rule
leap(year) = (year mod 4 = 0 AND year mod 100 ≠ 0) OR year mod 400 = 0
leap years up to Y = ⌊Y/4⌋ − ⌊Y/100⌋ + ⌊Y/400⌋The second formula counts the leap years from year 1 to Y and is used for the range count; the list itself is produced by checking each year (capped at 400 entries). The weekday of 29 February comes from the calendar date, so the tool can show that the next leap day, 29 February 2028, is a Tuesday.
Why the century exception exists
A solar (tropical) year lasts about 365.2422 days. One leap day every four years gives 365.25 – the Julian calendar – which drifts by about three days every 400 years. Dropping three leap days per 400 years (the century years not divisible by 400) gives 365.2425 days, close enough that the calendar drifts by roughly one day in 3,000 years.
Years before 1583 are accepted for curiosity, but the Gregorian rule is applied retroactively (a “proleptic” calendar); historically those years followed the Julian rule, and countries switched between 1582 and 1923.
Sources: Leap year – Gregorian and Julian rules, Tropical year length, Gregorian calendar – adoption by country.
Questions
- Is 2026 a leap year?
- No. 2026 ÷ 4 leaves a remainder of 2, so February 2026 has 28 days. The next leap year is 2028, the previous one was 2024.
- Why was 2000 a leap year but 1900 was not?
- Both are century years, so the ÷4 rule alone is not enough. 2000 is divisible by 400 and keeps its leap day; 1900 is not and loses it. The same will happen to 2100, 2200 and 2300, while 2400 will be a leap year.
- How many leap years are there in 100 years?
- Usually 24 or 25. A century such as 1901–2000 has 25 because it ends on a year divisible by 400; 1801–1900 has 24 because 1900 is skipped.
- What happens to people born on 29 February?
- They have a real birthday only every four years. In common years most celebrate on 28 February or 1 March; legal conventions differ by country. The Age Calculator counts the anniversary on 28 February in common years.