Leap Year Calculator

How to use the leap year calculator

Enter a year and the tool tells you whether it is a leap year, which of the rules decided it, how many days February and the year contain, and where the year sits between the leap years either side of it. The example loaded is 2026, which is not a leap year: it is not divisible by 4, so February keeps 28 days and the year runs to 365.

The last leap year before it was 2024 and the next one is 2028. Nothing here needs a locale or a calendar setting, because the leap rule is a property of the Gregorian calendar itself and is the same wherever that calendar is in civil use.

1. 1

   Divisible by 4: a leap year

   This first clause is the whole rule most people carry in their head, and on its own it is right about 96 percent of the time.

2. 2

   Divisible by 100: not a leap year, despite passing the first test

   That is why 1900 was a common year with no 29 February, and why 2100 will run 365 days.

3. 3

   Divisible by 400: a leap year after all, overriding the second

   That is why 2000 was a leap year even though it is a century year, and why 2400 will be one too.

The leap day itself has a weekday like any other date, and that is often the actual question behind the lookup.

Anyone born on 29 February meets a related problem: in a common year the date does not exist, and the convention most jurisdictions use for legal age is to treat 28 February as the anniversary, which keeps the birthday inside its own month. Practice varies for other purposes, and where a rule matters legally, such as when a fixed-term agreement expires, the document itself settles it rather than a calendar convention.

The eight-year gap across a skipped century

The gap between consecutive leap years is four years almost always, and the exception is worth knowing because it is the longest gap the calendar contains. Across a skipped century the gap stretches to eight: 2096 is a leap year, 2100 is not, and the next leap day after 29 February 2096 is 29 February 2104, which is 2,921 days later rather than the usual 1,461.

Nobody reading this will be inconvenienced by that particular gap, but the same arrangement happened between 1896 and 1904, and it is the reason software written on the assumption that every fourth year is a leap year has failed at century boundaries more than once.

The formula, worked line by line

The test is three integer divisions, applied in order, and the order is what makes it correct. Divisibility by 400 wins outright. Failing that, divisibility by 100 rules the year out. Failing both, divisibility by 4 rules it in.

Written the other way round, as a chain of exceptions, the same rule reads: every fourth year is a leap year, except century years, except those centuries divisible by 400. Both readings give identical answers on every year, and the tool reports which clause fired so the verdict can be checked by hand.

The reason the rule is shaped this way is astronomical rather than arbitrary. A tropical year, the time from one March equinox to the next, is roughly 365.2422 days.

Adding a day every four years, as the Julian calendar did, treats the year as 365.25 days and overshoots by about 11 minutes a year, which accumulates to a full day roughly every 128 years. Dropping three leap days every four centuries brings the average to 365.2425 days, an error of about 27 seconds a year, or a day in roughly 3,200 years.

leap = (year mod 4 = 0 and year mod 100 is not 0) or year mod 400 = 0
February days = 29 when leap, otherwise 28
days in year  = 366 when leap, otherwise 365
leap years in any 400-year cycle = 100 - 4 + 1 = 97
mean Gregorian year = (303 x 365 + 97 x 366) / 400 = 365.2425 days

The eight-year gap is the only irregularity in the whole sequence and it is worth spelling out. Consecutive leap years are normally 1,461 days apart, which is four years of 365 plus the extra day. Across a skipped century the interval doubles to 2,921 days: 29 February 2096 to 29 February 2104 spans eight years because 2100 is not a leap year.

The Gregorian reform that produced all of this was promulgated by the papal bull Inter gravissimas in February 1582 and took effect that October, when 10 days were dropped from the calendar. Adoption was staggered by country: Britain and its colonies did not switch until September 1752, dropping 11 days, and Russia not until 1918.

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