π Leap Year Calculator
Check any year against the full Gregorian rules.
Leap year?
How to Use This Calculator
Enter any year and hit Check Year. The headline answers yes or no; the details name the rule that decided (divisible by 4, century exception, or 400 exception), show February's day count, and list nearby leap years. The nearby-years list is the practical part: it answers "when is the next one" without further clicks.
Enter the year. Any year from 1 to 9999 works.
Hit Check Year and read the yes/no headline.
Read which rule applied, useful for century years that surprise people.
Use the nearby leap years list to plan around February 29.
The Method: Three Rules in Order
The rules nest: start with divisibility by 4, then carve out centuries, then restore the 400s. The order matters because 2000 passes all three tests and only the last one saves it. The astronomical target is the mean tropical year of about 365.2422 days; the Gregorian rules average 365.2425, an error of one day every 3,300 years, which is why nobody alive needs to worry about the next correction. The Julian calendar's simpler every-4-years rule averaged 365.25, drifting a day every 128 years, which is exactly the drift Pope Gregory XIII corrected in 1582 by deleting 10 days.
Worked Example
Check 2026: 2026 / 4 = 506.5, not whole, so Rule 1 fails: not a leap year. Check 2000: divisible by 4 (yes), divisible by 100 (yes, century), divisible by 400 (yes): leap year by Rule 3. Check 1900: divisible by 4 and 100, not by 400: not a leap year by Rule 2. Check 2028: divisible by 4, not a century: leap year. Enter each above and the calculator names the deciding rule every time. The pattern to internalize: centuries are guilty until proven innocent by 400.
Why February Gets the Extra Day
February is the shortest month because of Roman calendar politics: it was the last month of the old Roman year, where intercalary days were inserted, and it kept the role when January and February moved to the front. The Gregorian reform kept the tradition. Some proposals for calendar reform suggest adding the leap day to June or December instead, but 400 years of software, contracts, and habit make February 29 permanent. Leap seconds, the modern equivalent, get added to June or December instead, which confuses everyone equally.
Leap Years in Real Life
Software treats February 29 as a minefield: date libraries handle it, hand-rolled date math often does not, and the famous Excel 1900 bug treated 1900 as a leap year it was not. Contracts spanning February 29 need explicit terms for annual anniversaries. Payroll systems prorate salaries over 366 days in leap years, slightly lowering daily rates. Birthday systems need a defined behavior for February 29 births in common years. And every four years, the extra day shifts weekday patterns for the whole year, which is why perpetual calendars exist. Check the years that matter to your systems with the calculator; the century cases are the ones that bite.
Why the Gregorian Reform Happened
The Julian calendar's simple every-4-years rule overcorrected: the solar year is 365.2422 days, not 365.25, so the calendar gained about 11 minutes a year. By 1582 the drift totaled 10 days, pushing the spring equinox (and Easter's calculation) noticeably early. Pope Gregory XIII's fix dropped 10 days outright (October 4 was followed by October 15) and added the century exception: centuries leap only when divisible by 400. That trims the average year to 365.2425 days, leaving an error of about 26 seconds per year, or one day per 3,300 years. Protestant countries resisted for centuries: Britain and its colonies adopted the reform in 1752, dropping 11 days and sparking (apocryphal) riots over "stolen" days. The lesson for date math: proleptic Gregorian calculations before 1582 are a modern convention, not what people at the time experienced.
Leap Seconds: The Other Correction
Leap years fix the calendar's year length; leap seconds fix the clock's day length. Earth's rotation is gradually slowing and wobbles unpredictably, so Coordinated Universal Time (UTC) occasionally inserts a 61-second minute to stay aligned with the Sun. Unlike leap years, leap seconds are irregular: 27 were added between 1972 and 2016, then none for years. They are announced only months ahead, which makes them a headache for computing: the famous 2012 leap second crashed major websites whose software assumed minutes have 60 seconds. The trend is toward abolishing them: in 2022 the international timekeeping community voted to stop adding leap seconds by 2035, letting UTC drift from solar time instead. For everyday date math, leap seconds are invisible; for programmers and astronomers, they are a permanent footnote.
Second Worked Example: The Century Trap
Is 1900 a leap year? Apply the three rules in order. Divisible by 4? Yes, 1900 Γ· 4 = 475. Divisible by 100? Yes, so the century rule overrides: not a leap year unless also divisible by 400. 1900 Γ· 400 = 4.75, not whole, so 1900 was a common year with 28 days in February. Now try 2000: divisible by 4, divisible by 100, and 2000 Γ· 400 = 5 exactly, so 2000 was a leap year. This is the trap the calculator exists for: everyone remembers "divisible by 4" and forgets the century exception, which is exactly what broke so much software in 2000.
The 400-Year Cycle: Why the Calendar Repeats
The Gregorian calendar runs on a 400-year cycle containing exactly 146,097 days: 303 common years and 97 leap years. That total divides evenly by 7 (20,871 weeks), which means the entire calendar, weekdays included, repeats every 400 years. January 1, 2000 was a Saturday; January 1, 2400 will be a Saturday too. The 97 leap years per cycle give an average year of 365.2425 days, against the true solar year of about 365.2422 days: an error of one day every 3,300 years. Good enough that no living person needs to worry about the next reform.
Software Bugs: The Leap Years That Broke Things
Leap years have a body count in software. Microsoft Excel famously treats 1900 as a leap year, a deliberate bug kept for compatibility with Lotus 1-2-3, which means Excel's date math is wrong for dates before March 1, 1900. On December 31, 2008, millions of Zune media players froze simultaneously because of a leap-year bug in the device's clock driver: the code could not handle day 366. Programmers now treat February 29 as a mandatory test case, alongside time zones and daylight saving. If your birthday is February 29, you have likely broken at least one website's date validator, and you are entitled to feel smug about it.
Leap Day Babies: The Odds
The chance of being born on February 29 is about 1 in 1,461, the number of days in a 4-year leap cycle. Roughly 5 million people worldwide share the birthday, which sounds like a lot until you divide by 8 billion. Legally, most jurisdictions treat March 1 as the birthday in non-leap years, though some use February 28; the difference matters exactly once, for 18th and 21st birthdays. Leapling clubs exist in several countries, because any group this exclusive organizes.
Frequently Asked Questions
Is 2026 a leap year?
No. 2026 divided by 4 is 506.5, not a whole number, so February has 28 days. The next leap year is 2028. Enter any year above to check it with the full rule: divisible by 4, except centuries unless divisible by 400.
What are the leap year rules?
Three rules in order: divisible by 4 is a leap year, except century years (divisible by 100) are not, except centuries divisible by 400 are. So 2024 is a leap year, 1900 was not, and 2000 was. The calculator applies all three rules and tells you which one decided.
Why do we have leap years?
Because a year is about 365.2422 days long, not 365. Without correction, the calendar would drift about 6 hours a year, and after a century, seasons would shift by 24 days. Adding February 29 every 4 years (minus the century exceptions) keeps the calendar aligned with Earth's orbit.
Was 2000 a leap year?
Yes. Century years are normally not leap years, but 2000 is divisible by 400, which restores it. This surprises people because 1900 was not a leap year. The 400-year exception exists to fine-tune the average year length to 365.2425 days, very close to the true 365.2422.
Was 1900 a leap year?
No. It is divisible by 100 but not by 400, so the century rule excludes it. Anyone doing date math across 1900 must account for a missing February 29 that the simple "every 4 years" rule would wrongly include. Spreadsheet date systems famously inherited a related bug about 1900.
When is the next leap year?
2028. Leap years come every 4 years: 2024, 2028, 2032, 2036. Enter any year above and the calculator lists nearby leap years in both directions, so you can find the next or previous one instantly. The 4-year rhythm holds even for century years divisible by 400, like 2000, which was a leap year.
What happens to February 29 birthdays?
In common years, leap-day babies celebrate on February 28 or March 1; legally, most jurisdictions treat March 1 as the birthday for age purposes. Their actual birthdays still come every 4 years, which makes the 21st birthday land on the 5th or 6th celebration depending on counting method.
Do other calendars have leap years?
Yes, most solar calendars need them. The Hebrew calendar adds a leap month 7 times in 19 years; the Islamic lunar calendar adds a leap day to keep months aligned. The Gregorian system's 4/100/400 rule is just one solution to the universal problem that orbits do not divide into whole days.