What Is a Leap Year? The Rule, the Reason and the Next Ones

A leap year is a year with 366 days instead of 365, the extra day being 29 February. A year is a leap year if it divides exactly by 4, except century years (1900, 2100), which are leap years only if they also divide by 400. So 2024 was a leap year, 2026 and 2027 are not, the next one is 2028, and 2100 will not be one even though it divides by 4.

Check any year with the free leap year checker. Below is why the rule has that odd century exception, a list of leap years to 2100, what happens to people born on 29 February (including the law in India), and how leap seconds differ.

The rule in three questions

Ask these in order and stop at the first answer that settles it:

1. Does the year divide by 4? If not, it is a common year (365 days).
2. Does it divide by 100? If not, it is a leap year.
3. Does it divide by 400? If yes, it is a leap year; if not, it is a common year.

A quick test for the first step: a number divides by 4 if its last two digits do. 2028 ends in 28, and 28 ÷ 4 = 7, so it passes.

Why leap years exist

The seasons repeat every tropical year, about 365.2422 days. A 365-day calendar falls behind by almost 6 hours a year, which adds up to a day every four years and about 24 days a century. Left alone, festivals and harvest dates tied to the calendar would slowly drift through the seasons.

Step 1: the Julian calendar, a day every four years

The calendar introduced under Julius Caesar in 45 BC added one day every fourth year, giving an average of 365.25 days. That is about 11 minutes a year too long: 365.25 − 365.2422 = 0.0078 days, or 11.2 minutes. Small, but it adds up to a full day roughly every 128 years. By the 1500s the spring equinox had drifted about 10 days earlier on the calendar.

Step 2: the Gregorian fix, skip three leap days every 400 years

In 1582 Pope Gregory XIII's reform removed 10 days at once (Thursday 4 October 1582 was followed by Friday 15 October) and added the century rule. Over 400 years there are now 97 leap years instead of 100:

• Average year = 365 + 1/4 − 1/100 + 1/400 = 365 + 0.25 − 0.01 + 0.0025 = 365.2425 days.
• Error against the tropical year = 365.2425 − 365.2422 = 0.0003 days, about 26 seconds a year.
• That is roughly one day in about 3,000 years. Published estimates range from about 3,000 to 3,300 years, depending on which value of the year is used.

Britain and its colonies switched only in 1752, when 2 September was followed by 14 September. India's national calendar, the Saka calendar, follows the same leap years: 1 Chaitra falls on 22 March in a common year and on 21 March in a leap year, when Chaitra has 31 days instead of 30.

Leap years from 2024 to 2104

Every fourth year is a leap year until the century exception at 2100:

2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052, 2056, 2060, 2064, 2068, 2072, 2076, 2080, 2084, 2088, 2092, 2096, then not 2100, then 2104.

So there is an eight-year gap from 2096 to 2104. The next leap day, 29 February 2028, is a Tuesday; the one after, 29 February 2032, is a Sunday. A leap year also changes how many working days a year has, which is covered in how many working days are in a year.

Using the leap year checker

Type a year into the leap year checker and press Check year. It tells you whether the year is a leap year, whether it has 365 or 366 days, whether February has 28 or 29 days, and the next leap year after it. Some results:

• 2026: not a leap year, 365 days, February has 28 days, next leap year 2028.
• 2096: a leap year, 366 days, next leap year 2104 (not 2100).
• 2100: not a leap year, next leap year 2104.

Behind the result is one line of code, the same test programmers use: (y % 4 === 0 && y % 100 !== 0) || y % 400 === 0. The common bug is checking only y % 4, which wrongly makes 1900 and 2100 leap years. Microsoft Excel has a famous version of this: for compatibility with the old Lotus 1-2-3 spreadsheet, it treats 1900 as a leap year and accepts 29 February 1900 as a date.

Two limits of the checker. It applies the Gregorian rule to every year, including years before the calendar was adopted, so it says 1500 was not a leap year, although under the Julian calendar used at the time, 1500 did have a 29 February (as did 1700 in Britain). And it needs a whole, non-zero year: 0 gives "Enter a year", and decimals are cut to the whole year.

Born on 29 February

A leap day comes once in 1,461 days (four years), so roughly 1 person in 1,461 is born on it. In common years most "leaplings" celebrate on 28 February or 1 March.

The law in India. Section 3 of the Majority Act, 1875 says a person domiciled in India attains majority on completing 18 years. In counting age, the day of birth counts as a whole day, and the person is "deemed to have attained majority at the beginning of the eighteenth anniversary of that day". The Act has no special rule for someone born on 29 February, whose eighteenth anniversary will usually fall in a common year with no such date, and we found no Indian court ruling settling it. The two conventions in use are 28 February and 1 March. If a date matters, for an exam age limit, a driving licence or a bank account, follow what the authority that sets the limit says.

What the age calculator shows. The age calculator treats 28 February as the birthday in common years. For someone born on 29 February 2008:

Exam notices usually state a cut-off date such as "born on or after 1 March 2004"; how to read those is in age calculator for exam cut-off dates. For the exact number of days between two dates, use the date difference calculator.

Leap seconds are a different thing

Leap years keep the calendar in step with the seasons. Leap seconds kept clock time (UTC) in step with the Earth's rotation, which is slightly irregular. Since 1972, 27 leap seconds have been added, the last at the end of 31 December 2016.

They are on their way out. In November 2022 the General Conference on Weights and Measures (CGPM) decided that the maximum allowed difference between Earth-rotation time and UTC "will be increased in, or before, 2035", which in practice ends leap seconds for at least a century. Its meeting of 13 to 15 October 2026 produced a follow-up resolution, "On the technical actions needed to ensure the continuity of UTC". Unix time, which most computers use, already ignores leap seconds; see how Unix timestamps work.

Frequently asked questions

Is 2026 a leap year?

No. 2026 does not divide by 4, so it has 365 days and February has 28. The next leap year is 2028.

Why is 2100 not a leap year?

Century years must divide by 400 to be leap years. 2100 ÷ 400 = 5.25, so it is a common year, like 1900. 2000 was a leap year because 2000 ÷ 400 = 5.

How many days are there in a leap year?

366, with 29 days in February. A common year has 365.

How often is there a leap year?

Every four years, except three times in every 400 years when a century year is skipped. That gives 97 leap years in each 400-year cycle.

When does someone born on 29 February turn 18 in India?

The Majority Act, 1875 does not address it directly. Majority begins at the start of the 18th anniversary of the birth date; in a common year people use 28 February or 1 March. For an official purpose, ask the authority concerned.

Sources: Papal bull Inter gravissimas (1582); Calendar (New Style) Act, 1750 (Great Britain); Know India, Government of India, "National Calendar"; The Majority Act, 1875, section 3; BIPM, Resolution 4 of the 27th CGPM (2022) and the 28th CGPM (October 2026); Microsoft, "Excel incorrectly assumes that the year 1900 is a leap year".