A leap year list is a sequence of years that contain 366 days instead of the usual 365, with February gaining a 29th day to keep the Gregorian calendar aligned with Earth's orbit around the Sun. You build one by applying a single three-part rule: a year is a leap year when it is divisible by 4; it is also a leap year if it is a century year ending in 00 but is additionally divisible by 400; in every other case, it is a common year of 365 days. Because of that last test, an accurate leap year list never simply includes every fourth year. Century years that fail the 400 check are skipped, leaving exactly 97 leap years in any continuous 400-year span. This is why 2000 sits inside a complete leap year list while 1900 and 2100 do not, even though all three are divisible by 4. To build a leap year list for any range without doing the arithmetic by hand, the free Leap Year Calculator instantly shows the leap years immediately before and after any year you enter.
The leap year rule behind every accurate list
The Gregorian calendar decides leap years with one arithmetic rule that has three parts, and understanding all three is the only way to build a correct leap year list. First, if a year is not divisible by 4, it is a common year of 365 days, so 2023 and 2025 are not leap years. Second, if a year is divisible by 4 but is not a century year, it is a leap year of 366 days, so 2024 and 2028 are leap years. Third, century years need the stricter test: 1900 and 2100 are divisible by 4 but not by 400, so they are common years, while 1600, 2000 and 2400 are divisible by 400 and are leap years. The rule can be written in one line as: divisible by 4 AND (not divisible by 100 OR divisible by 400).
That single extra test is what changes the size of a leap year list. Without it, every year ending in 00 would also be a leap year, and you would get 25 leap years per century instead of 24 or 25. With it, three of every four century years are skipped, and the calendar averages the correct number of days per year. Over a 400-year span the rule produces exactly 97 leap years and 303 common years, for a total of 146,097 days, the same as 400 times 365.2425, the average length of the Gregorian calendar year.
How to calculate a leap year list step by step
Use this procedure to assemble a leap year list for any start and end year. The method is the same whether you are listing the next ten leap years, the leap years of the 1900s, or the leap years in a 400-year block.
- Decide on the range you want the list to cover and write down every whole-number year from the start year to the end year.
- For each year, check whether it is divisible by 4 by computing year ÷ 4 and confirming there is no remainder. If there is a remainder, mark the year as a common year and skip it.
- For years that pass the ÷ 4 test, check whether the year is a century year, meaning whether it ends in two zeros, such as 1800, 1900, 2000, or 2100.
- If the year is not a century year, add it to your leap year list now; it has 366 days and February has 29 days.
- If the year is a century year, perform one more check: divide it by 400 and confirm there is no remainder.
- Add century years to the list only when they pass the ÷ 400 test. Century years that pass ÷ 4 but fail ÷ 400 stay off the list.
- Cross out every year that failed any check. The remaining years form your completed leap year list, with the days in February and the total days in each year ready to be reported alongside.
Worked example for 2024: 2024 ÷ 4 = 506 with no remainder, so it passes the first test. 2024 does not end in 00, so it is not a century year and the stricter ÷ 400 test does not apply. Result: 2024 is a leap year, the year has 366 days, and February has 29 days.
Read the leap year list from the calculator
The Leap Year Calculator applies the three-part rule for you and surrounds every answer with a small built-in leap year list, so you do not have to run the divisibility checks by hand. Type any year into the field; it defaults to the current year, so you see an answer right away. The result tells you whether the year is a leap year, the rule that decided it, and the number of days that year has (365 or 366). Below the result, the tool lists the previous and next leap years plus the leap years near your chosen year, giving you a leap year list segment without any extra steps.
Everything runs in your browser, so nothing is uploaded and there is no signup or limit on how many years you check. The tool works for any whole-number year, including 0 and negative years under the proleptic Gregorian calendar, where astronomical year numbering treats year 0 as 1 BC, because the divisibility rule holds for every integer.
Century years that break the every-four-years pattern
The simplest way to see why a "divisible by 4" shortcut produces the wrong leap year list is to look at a table of century years. The table below shows every century year from 1600 through 2400 and the rule outcome for each.
| Year | Divisible by 4? | Century year? | Divisible by 400? | Leap year? |
|---|---|---|---|---|
| 1600 | Yes | Yes | Yes | Yes (366 days) |
| 1700 | Yes | Yes | No | No (365 days) |
| 1800 | Yes | Yes | No | No (365 days) |
| 1900 | Yes | Yes | No | No (365 days) |
| 2000 | Yes | Yes | Yes | Yes (366 days) |
| 2100 | Yes | Yes | No | No (365 days) |
| 2200 | Yes | Yes | No | No (365 days) |
| 2300 | Yes | Yes | No | No (365 days) |
| 2400 | Yes | Yes | Yes | Yes (366 days) |
Out of those nine century years, only three are leap years: 1600, 2000, and 2400. The other six pass the ÷ 4 test but fail the ÷ 400 test and are skipped. If you had built your leap year list with a simple "every fourth year" rule, you would have wrongly included 1700, 1800, 1900, 2100, 2200 and 2300.
Why people generate a leap year list
A short, accurate leap year list has many everyday uses beyond satisfying curiosity. People planning a February 29 birthday or anniversary need the next leap year on the calendar so they know when the next celebration falls, and a list of upcoming leap years makes that trivial to scan. Anyone calculating an exact age or a date difference that crosses a leap day needs to know whether February contributed an extra day to the total. Payroll, billing, and subscription cycles sometimes anchor on calendar days, and listing the leap years in a window helps confirm that a recurring payment has been counted correctly. Developers, analysts, and spreadsheet users also build leap year lists to validate dates, detect off-by-one errors, and settle trivia such as whether 2100 is a leap year.
For planning around February 29 birthdays and anniversaries specifically, see the guide on how to calculate leap year birthday dates.
Related tools for date and time arithmetic
The Leap Year Calculator tells you whether a year has 365 or 366 days, but date math usually needs a little more. To count the exact days between two specific dates, use a date difference calculator. To find which numbered day of the year a date falls on, useful for shipping manifests, project deadlines, and batching, use a day-of-year calculator. To work out an exact age across multiple years that may include a leap day, use an age calculator. Pairing a leap year list with these tools gives you a complete way to reason about calendar dates in any range.
For a deeper look, see How to Calculate the Week Number of Any Date in a Year.