Contents
How the calculator works
A Gregorian leap year has 366 days and includes February 29. Enter a positive whole-number year to check it; the result also states whether the year is leap or common.
The rule needs three divisibility checks
A year is leap when it is divisible by 400, or when it is divisible by 4 but not by 100.
This is the rule used by the Gregorian calendar. It places 97 leap days in each 400-year cycle, giving an average calendar year of 365.2425 days. The US Naval Observatory explains the rule and its calendar background.
Read the result in two parts
The large number is the length of the selected year: 366 for a leap year and 365 for a common year. The smaller label gives the classification. The calculator accepts positive integer year numbers and applies the Gregorian rule even to years before the calendar was introduced.
Century years reveal the important exception
- 2024: divisible by 4 and not by 100, so it is a leap year with 366 days.
- 1900: divisible by 100 but not by 400, so it is a common year with 365 days.
- 2000: divisible by 400, so it remains a leap year with 366 days.
The shortcut “every four years” works for many nearby dates but fails at most century boundaries. The 100 and 400 checks are what make 1900 and 2000 different.
Historical dates need a calendar choice
The Gregorian calendar was introduced in 1582, and different places adopted it at different times. For a historical date, first identify which calendar the record used. The Julian calendar follows a different leap-year rule, while other civil and religious calendars may add days or months in other ways.
This tool deliberately answers one narrow question: how the entered year is classified under the Gregorian rule. It does not reconstruct a country’s historical calendar.
Questions about leap years
These answers cover the exceptions that most often cause a wrong result.
Is every year divisible by 4 a leap year?
No. A century year must also be divisible by 400, so 1900 was common while 2000 was leap.
Why does a leap year have 366 days?
The Gregorian calendar adds February 29, increasing February to 29 days and the year to 366.
Will 2100 be a leap year?
No. It is divisible by 100 but not by 400, so the calculator returns 365 days.
Can I check a year before 1582?
Yes, but the result is a proleptic Gregorian classification. A historical document may have used another calendar.
Does the rule work for other calendars?
No. The calculator implements only the Gregorian 4, 100, and 400 divisibility rule.
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