October 26, 2026 6 min read

Leap Year Rules: How Leap Years Work and Why 2100 Will Not Be One

A leap year is a year with 366 days instead of 365, the extra day being February 29. The rule most people know, "every four years", is only the first of three parts. The full Gregorian rule also skips the leap day in most century years, which is why 2000 was a leap year, 1900 was not, and 2100 will not be either. The next leap day is Tuesday, February 29, 2028.

The rule in three lines

1. If the year is divisible by 4, it is a leap year.
2. Except if it is also divisible by 100: then it is not.
3. Except if it is also divisible by 400: then it is a leap year after all.

Run any year through those lines in order and stop at the last one that applies. 2028 is divisible by 4 and not by 100, so it is a leap year. 2100 is divisible by 4 and by 100 but not by 400, so it is a common year. 2000 passes all three, so it was a leap year. The US Naval Observatory states the rule in exactly these terms, with 1700, 1800 and 1900 as the standard examples of century years that were not leap years.

In a spreadsheet, with the year in cell A1, this formula does the whole test: =IF(OR(MOD(A1,400)=0,AND(MOD(A1,4)=0,MOD(A1,100)<>0)),"Leap","Common"). Most programming languages have it built in, for example calendar.isleap() in Python.

Why the rule exists

The Earth takes about 365.2422 days to go from one spring equinox to the next. Royal Museums Greenwich uses that figure for the solar (tropical) year; in clock terms it is 365 days, 5 hours, 48 minutes and 46 seconds. A calendar of 365 whole days falls behind by almost six hours every year, and after a century the seasons would arrive about 24 days late.

The Julian calendar, introduced in 45 BC, fixed that with a leap day every four years. That makes the average year 365.25 days, which overshoots by about 11 minutes. Eleven minutes a year is a day every 128 years or so, and by 1582 the spring equinox had slid about 10 days earlier in the calendar.

The Gregorian reform of 1582 dropped those 10 days and added the century exception. Removing three leap days every 400 years leaves 97 leap years per cycle and an average year of 365.2425 days. That is still about 26 seconds too long, but it adds up to a full day only after roughly 3,200 years. The whole story of the reform, and of the countries that took centuries to adopt it, is in our guide to how the Gregorian calendar works.

Upcoming leap years

Every year divisible by 4 from 2028 to 2096 is a leap year, with no exceptions in between. The table lists them with the weekday of each February 29, since that is the day leap-year birthdays, contracts and payroll actually land on.

Then comes the gap. 2100 is not a leap year, so 2096 and 2104 are eight years apart, the first such gap since 1896 to 1904. 2200 and 2300 skip their leap days too; 2400 keeps it. Anyone writing software today that has to handle dates past 2099 should test 2100 explicitly, because "divisible by 4" code passes every test until then.

Counted from October 26, 2026, the next leap day is 491 days away. The countdown timer will count to it, or to any other date, for you.

February 29 birthdays

People born on February 29 are often called leaplings or leapers. With one leap day in every 1,461 days of a four-year cycle, the chance that a given birthday falls on February 29 is about 1 in 1,461. Over the full 400-year cycle, which drops three leap days, it is closer to 1 in 1,506.

In common years, leaplings have to pick a day. Most celebrate on February 28, keeping the birthday in February, or on March 1, the day after February 28, which is when the full year since their last birthday has passed. Neither is wrong for a party.

For law and paperwork there is no single answer; it depends on the jurisdiction and sometimes on the purpose. If an exact date matters, such as for a driving test, a retirement date or a policy that turns on reaching an age, check the rule that applies to you rather than assuming. Our age calculator counts exact years, months and days from any date of birth, including February 29, and the method behind it is in how to calculate age.

Leap day vs leap second

They sound alike and solve different problems. A leap day keeps the calendar aligned with the Earth's orbit around the Sun. A leap second keeps atomic clock time (UTC) aligned with the Earth's rotation on its axis, which is slightly irregular. Leap days follow a fixed rule you can apply centuries ahead; leap seconds were announced about six months in advance after astronomers measured the Earth's spin.

Twenty-seven leap seconds were added between 1972 and 2016, and none since. In 2022 the General Conference on Weights and Measures decided to widen the allowed gap by 2035, which is meant to keep UTC free of leap seconds for at least a century. Leap days are not. The details are in leap seconds and why they are ending.

Check any date

The date calculator applies the full 4, 100 and 400 rule, so counting days across February in a leap year, or across 2100, gives the right answer. For how leap years affect the number of weeks in a year (and why some years have 53 ISO weeks), see how many weeks are in a year.

Frequently asked questions