Leap Year Calculator (Is It a Leap Year?)

Type two years and this tool lists every leap year between them, with a running total and the exact number of February 29s in that span. That is the part almost every leap year checker skips. They give you yes or no for a single year and stop. You can still do that single check here, enter a year and get a straight verdict plus the reason it landed that way, but the range list is the reason to bookmark this one. How many leap years are in the 21st century, every leap year from 1900 to 2100, the count of Feb 29s a billing cycle will cross. It all runs in your browser, and the only thing a share link carries is the years you asked about.

The rule itself is short. A year is a leap year if it divides by 4, except years that divide by 100, unless they also divide by 400. So 2024 is a leap year, 1900 is not, and 2000 is. There are three tiers, and most people stop at the first one. That first tier is right 97 times out of 100, the century years are where it breaks. Of the 100 years in a 400 year run that divide by 4, three are skipped centuries and the fourth century comes back, so the rule misses three times.

1. Check a year. Leave the tool on "Check a year," type any whole year, and read the verdict. You get a plain yes or no, the days in that year (365 or 366), whether February has 28 or 29, and which part of the rule decided it.
2. Or tap Use this year to drop in the current year.
3. Switch to List a range for the count. Enter a start year and an end year. The tool shows the total leap years, the number of February 29 days, the years spanned, and the common (non-leap) years, then draws the leap years themselves. Past 1,200 of them it draws the first 1,200 and says so on the panel, and the copy button still carries the whole set.
4. Tap any listed year to jump back to the single check for it, so you can see the reason and the leap years around it.
5. Copy the list with one button, which hands over every leap year in the range even when the panel only had room to draw the first 1,200, or use Share to send someone the exact range as a link. Teachers and bloggers can Embed the live tool with a range already set.

Change any field and the numbers update on the spot. Try 2000 to 2100 and you get 25 leap years, since the century year 2100 is one the rule skips.

This is the feature the free single-year checkers never bother building, and it answers a surprising number of real questions. How many leap years since I was born. How many February 29s fall inside a five-year loan. Which years in a genealogy range could hold a leap-day birth. A teacher building a worksheet wants the full list from 1900 to 2000, not one year checked at a time.

The count is exact rather than estimated. Over any 400-year block there are 97 leap years, so the long-run average is close to 24.25 per century. But you cannot assume 25 a century and be right, because the century years break the pattern. From 2000 through 2099 there are 25 leap years. From 1900 through 1999 there are only 24, because 1900 was common. The tool does the real counting, so a range that crosses a skipped century still comes back correct.

One detail is worth knowing here. The gap between two leap years is usually four years, but around a skipped century it stretches to eight. 1896 was leap, 1900 was not, 1904 was leap. That eight-year gap is the longest you will ever see, and it only happens three times in four hundred years.

If a leap-year mistake shows up in code, a spreadsheet, or a bar argument, it is almost always a century year. The simple "divisible by 4" shortcut says 1900 and 2100 are leap years, and neither one actually is.

Here is the run for the centuries around us:

• 1600, leap (divides by 400).
• 1700, 1800, 1900, all common.
• 2000, leap. This is the one that fooled a lot of software.
• 2100, 2200, 2300, all common.
• 2400, leap.

That is three common century years, then one leap. The year 2000 hid a real bug for years. A programmer who knew to skip century years but forgot the 400 override would mark 2000 as common, which is wrong, and that shortcut bit real systems in the Y2K run-up. This tool applies all three tiers, so 2000 comes back leap and 1900 comes back common, every single time.

The Earth takes about 365.2422 days to circle the Sun, not a clean 365.25. Adding a day every four years assumes a flat quarter-day, which overshoots slightly. Over 400 years that small excess piles up to roughly three days. The century rule removes those three surplus days: skip the leap day in 1700, 1800, and 1900, keep it in 2000. That pulls the average down to 365.2425 days a year, a hair over the true figure. The leftover error is about a day in 3,200 years, which nobody alive needs to plan around.

The whole point of a leap year is that one extra day, February 29. February sits at 28 days most years and stretches to 29 in a leap year. People born on the 29th are called leaplings, and their birthday only lands on the calendar every four years. Roughly one person in 1,461 is one. In common years most of them celebrate on February 28 or March 1, and which one is part personal choice, part legal question, since jurisdictions differ on when the anniversary falls for things like ages of majority.

There is a US footnote here too. George Washington was born February 11 under the old Julian calendar, which became February 22 when the British colonies switched to the Gregorian calendar in 1752 and dropped eleven days, so he effectively had two birthdays. This tool always applies the modern Gregorian rule, even for years before it existed, which is the standard convention for calendar math and keeps every answer consistent.