How this calculator works
The Gregorian calendar rules are applied directly. A year divisible by 4 is a leap year, unless it is a century year that is not divisible by 400. That keeps the average year close to the tropical year by dropping three leap days every four centuries. February 29 is the added day.
Formula
leap = (year mod 4 = 0) and (year mod 100 is not 0 or year mod 400 = 0). A leap year has 366 days and other years have 365. The next leap year is the smallest leap year greater than the tested year.
Worked example
2026 is not a leap year and has 365 days; the next leap year is 2028. In the range 2024 to 2050 the leap years are 2024, 2028, 2032, 2036, 2040, 2044 and 2048, seven in all. 1900 was not a leap year because it is divisible by 100 but not by 400, while 2000 was.
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Frequently asked questions
Why is 1900 not a leap year but 2000 is?
The century rule removes leap days from years divisible by 100, but the 400 rule restores them for years divisible by 400. 1900 fails the 400 test and 2000 passes it.
Does this apply before the Gregorian reform?
The page uses the proleptic Gregorian calendar, which extends the same rules backward without the historical changeover from the Julian calendar. Dates before 1582 in Catholic countries are therefore given on a modern uniform calendar.
Is a leap year always 366 days long?
On the civil calendar, yes. If you measure the interval between the same clock time a year apart, clock changes such as daylight saving can make the elapsed time in hours differ from a simple multiple of 24.
Why add a leap day at all?
A tropical year is about 365.2422 days, not a whole number. Without leap days, the calendar would drift out of step with the seasons by roughly one day every four years.