At a glance
A leap year is a calendar year in which February has 29 days, which makes the year 366 days long instead of 365. You need the answer whenever you build a calendar, count the days in a span of time, set up a schedule that repeats yearly, or deal with anything pinned to February 29. This checker applies the Gregorian rule used by most of the world today (divisible by 4, except century years, except century years divisible by 400) and shows which step decided the result, so you can see the reasoning instead of just a yes or no.
Why leap years exist
The seasons repeat once every tropical year, the time the sun takes to return to the same point in the cycle of equinoxes and solstices. That interval is about 365.2422 days, not a whole number. If the calendar always used 365 days, it would fall behind by roughly a quarter of a day every year, close to six hours. After a few decades the dates would visibly drift against the seasons, and after a few centuries midsummer would land in a different month. Inserting an extra day at regular intervals keeps the average length of the calendar year close to the tropical year. The 365.2422 figure is an approximation; the true length changes very slowly over long periods.
The rule and its two exceptions
- If the year is divisible by 4, it is a leap-year candidate.
- If it is also divisible by 100, it goes back to being a common year.
- If it is also divisible by 400, it is a leap year after all.
Adding a day every fourth year alone gives an average year of 365.25 days, which is a little longer than the tropical year, so the calendar slowly runs ahead of the sun. The Gregorian rule trims that excess by dropping three leap years in every 400. The table below shows exactly where each century year is filtered, computed by the same function the checker uses.
| Year | Divisible by 4 | Divisible by 100 | Divisible by 400 | Result |
|---|---|---|---|---|
| 1600 | Yes | Yes | Yes | Leap year |
| 1700 | Yes | Yes | No | Common year |
| 1800 | Yes | Yes | No | Common year |
| 1900 | Yes | Yes | No | Common year |
| 2000 | Yes | Yes | Yes | Leap year |
| 2100 | Yes | Yes | No | Common year |
| 2200 | Yes | Yes | No | Common year |
| 2300 | Yes | Yes | No | Common year |
| 2400 | Yes | Yes | Yes | Leap year |
1700, 1800, 1900, 2100, 2200 and 2300 are common years, while 1600, 2000 and 2400 are leap years. Because 2000 was a multiple of 100 that still counted as a leap year, the shortcut "century years are never leap years" gets it wrong. The reverse shortcut "every fourth year is a leap year" fails for 1900 and 2100. Both exceptions have to be applied in order.
Leap years from 2024 onward
Running the rule forward gives these leap years: 2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052, 2056, 2060, 2064, 2068, 2072, 2076, 2080, 2084, 2088, 2092, 2096, 2104, 2108, 2112. Most gaps are four years, but 2096 and 2104 are eight years apart because 2100 is skipped. Anyone planning software or schedules that reach into the next century should not assume that 2100 follows the usual pattern.
How accurate is the 400-year cycle?
The Gregorian calendar repeats every 400 years, which contain exactly 146,097 days (a whole number of weeks, so even the weekdays repeat). The table compares it with the naive every-fourth-year rule, using 365.2422 days as the tropical year. Since that value is itself approximate, read the last column as an order of magnitude.
| Rule | Leap years per 400 | Days per 400 years | Average year (days) | Excess over tropical year (days/yr) | Years to drift one day |
|---|---|---|---|---|---|
| Every 4th year (naive rule) | 100 | 146,100 | 365.2500 | +0.0078 | about 128 |
| Gregorian rule | 97 | 146,097 | 365.2425 | +0.0003 | about 3,333 |
The naive rule would accumulate a full day of error in roughly 128 years, while the Gregorian rule holds out for thousands of years. It is still not perfect, so very distant future centuries could require another adjustment. Historically, the Gregorian calendar was introduced to correct the accumulated drift of the older Julian calendar, which used the plain every-fourth-year rule, and countries adopted it at very different times. This checker applies the Gregorian rule to every year you type, so for years before a region adopted it, the result describes a projected calendar and not the one people actually used.
Worked examples
- 2024: divisible by 4 and not by 100, so it is a leap year with 366 days and a 29-day February.
- 1900: divisible by 4 and by 100 but not by 400, so it is a common year. This is the classic trap next to 2000.
- Counting a century: 2000 to 2099 contains 25 leap years, giving36,525 days. 1900 to 1999 contains 24, giving36,524 days. A century is therefore not always 36,525 days.
- Odds of a February 29: in each 400-year cycle there are only 97 leap days, so a randomly chosen calendar day has about a 1 in 1,506 chance of being February 29.
Common mistakes and limits
- Remembering only "divisible by 4" gives wrong answers for 1900, 2100, 2200 and 2300.
- Remembering only "century years are not leap years" gives wrong answers for 1600, 2000 and 2400.
- Leap years belong to the solar (Gregorian) calendar. Lunisolar calendars insert whole leap months, which follow a different rule.
- This tool takes a year only. It does not check whether a particular date exists, such as February 30.
- A leap second, the occasional extra second added to clocks, is a separate correction for Earth's rotation and has nothing to do with leap years.
- Programming tip: write the test as
(y % 4 === 0 && y % 100 !== 0) || y % 400 === 0, and avoid shortcuts that only check divisibility by 4.
Putting the result to use
Use 365 days for a common year and 366 for a leap year when you need the length of a year. For interest or pro-rata calculations, contracts differ on whether they use a fixed 365-day basis or the actual number of days, so read the terms that apply to you. If a deadline, anniversary or renewal falls on February 29, the document decides whether a common year moves it to February 28 or March 1, and that varies between contracts and jurisdictions. For the exact number of days between two dates, theD-Day Countdown Calculator counts leap days automatically, and the Age Calculator handles birthdays, including February 29 ones.
Related tools
- D-Day Countdown Calculator - days remaining until a target date
- Age Calculator - exact age from a birth date
- Birthday Paradox Calculator - the odds of shared birthdays
- Unit Converter - convert time and other units
Frequently Asked Questions
How do people born on February 29 celebrate their birthday?
In years without a February 29, many people mark February 28 or March 1 instead. The date that counts officially depends on the country and on the rule or contract involved, so there is no single answer.
Does the lunar calendar have a similar concept?
Lunisolar calendars add a whole extra month every few years to stay in step with the solar year. That is a different mechanism from the Gregorian leap day and follows its own rules.
Why was 2000 a leap year but 1900 was not?
Both are multiples of 100, so both are first treated as common years. 2000 is also a multiple of 400, which turns it back into a leap year. 1900 is not divisible by 400, so it stays common.
Is 2100 a leap year?
No. It is a multiple of 100 but not of 400, so it is a common year, and the leap year after 2096 is 2104.
How many days and hours are in a leap year?
A leap year has 366 days, or 8,784 hours, and a common year has 365 days, or 8,760 hours. The Gregorian calendar averages 365.2425 days per year over its 400-year cycle.
Can I check years in the distant past or future?
The arithmetic works for any year from 1 up, but before a region adopted the Gregorian calendar the real calendar was different, so the answer is a backward projection of the current rule. This tool accepts whole years from 1 to 999999.