Leap Years: How They Work and Which Years Are Leap

Last reviewed on May 4, 2026

A leap year has 366 days instead of 365. The extra day is February 29. The rule that decides which years are leap is older than most people think, and it has two exceptions that almost everyone forgets — the kind of exceptions that bit a lot of software in the year 2000. This page explains the rule, lists the next leap years, and shows what changes when February has 29 days.

The rule

A year is a leap year when:

• It is divisible by 4, and
• It is not divisible by 100 — unless
• It is divisible by 400.

That last clause is the part most often missed. In short:

• 2024 — divisible by 4, not by 100 → leap.
• 2025 — not divisible by 4 → not leap.
• 2100 — divisible by 4 and by 100, but not by 400 → not leap.
• 2000 — divisible by 4, by 100, and by 400 → leap.

Why the rule exists

The Earth takes about 365.2422 days to orbit the Sun. A 365-day calendar would drift roughly a day every four years, so the Julian calendar added one leap day every fourth year — overshooting by about 11 minutes per year. By the 16th century the Julian calendar had drifted ten days off the seasons, which is why Easter kept moving away from the spring equinox. The Gregorian reform in 1582 dropped three leap years out of every 400 (the centurial-not-divisible-by-400 case) to bring the calendar back into alignment. The result is accurate to within about one day in 3,200 years.

Leap years from 2025 to 2045

Within the range covered by this site, the leap years are:

• 2028
• 2032
• 2036
• 2040
• 2044

Every other year in this range — including 2025, 2026, and 2027 — has 365 days. None of the years in this range hits the centurial exception; the next year that looks like a leap year but isn't is 2100.

What changes in a leap year

February has 29 days

The most visible change is on the February calendar. February 2028 will be the next time the Site shows a 29th column for February.

The day of the week shifts by one extra day

A non-leap year ends on the same day of the week it started; a leap year ends one day later. December 31, 2025 is a Wednesday, and so is January 1, 2026. December 31, 2028 is a Sunday but January 1, 2028 is a Saturday — because 2028 is a leap year, the calendar advances by two days between consecutive January 1s instead of one.

Some recurring dates land on a different weekday

Birthdays and anniversaries that fall after February jump forward by an extra day in a leap year compared with the year before. So your July 4 weekday in 2028 will not follow the simple "one weekday later than last year" pattern; it will be two days later.

February 29 birthdays

People born on February 29 — sometimes called "leaplings" — have an actual birthday once every four years. In other years, most legal systems treat their birthday as either February 28 or March 1, depending on the jurisdiction and the purpose (driver's licenses, alcohol-purchase age, pension eligibility). The choice matters for legal purposes; for personal celebrations, leaplings usually pick whichever date fits.

Leap seconds are different

A leap year is not the same thing as a leap second. Leap seconds are occasional one-second adjustments added to UTC to keep atomic time in sync with the Earth's slowing rotation. They are announced by the International Earth Rotation and Reference Systems Service and have nothing to do with the calendar rule above. As of late 2025, the international metrology community has agreed to phase out leap seconds by 2035.

Quick check: is this year a leap year?

1. Divide the year by 4. If there's a remainder, it's not a leap year.
2. If it divides evenly, divide by 100. If there's a remainder, it is a leap year.
3. If it divides evenly by 100, divide by 400. If there's a remainder, it's not a leap year. Otherwise, it is.

Common mistakes

• Treating "every four years" as the whole rule. That gives the wrong answer for centuries that aren't divisible by 400.
• Assuming the leap day is at the end of the year. It's the end of February, not the end of December.
• Building software that hard-codes "366 days = leap" instead of checking the date. February 29 is the only date that exists in some years and not others — every spreadsheet, contract template, and date picker eventually has to handle it.
• Confusing leap year with the ISO 8601 53-week year. They overlap sometimes, but not always — see the ISO week numbers page.

Related

• ISO 8601 week numbers — the other rule that surprises people every January.
• Working days vs calendar days — leap years quietly add a working day to most months in February.
• 2026 US federal holidays — see how a non-leap year arranges weekday holidays.