Leap Year Checker – Instantly Verify Any Calendar Year
Is 2028 a leap year? What about 1900 or 2100? The Leap Year Checker answers instantly for any year from 1 to 9999, showing not just a Yes/No answer but the exact divisibility rule that applied. It also scans year ranges, finds the nearest leap years before and after any given year, and reports the number of days in February and in the year overall.
The Gregorian Leap Year Rule
Under the modern Gregorian calendar, used by nearly every country today, a year is a leap year if it is divisible by 4 — except century years, which must also be divisible by 400 to qualify. This can be expressed as:
isLeapYear = (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0
This extra ÷100/÷400 exception exists because a 365.25-day year (the plain ÷4 rule) is very slightly longer than the actual solar year. Removing three leap days every 400 years keeps the calendar aligned with the seasons over long timescales.
Why 1900 Isn't a Leap Year, But 2000 Is
Both 1900 and 2000 are divisible by 4 and by 100, so the century exception kicks in for both. The deciding factor is divisibility by 400: 1900 ÷ 400 = 4.75 (not a whole number, so it's a common year), while 2000 ÷ 400 = 5 exactly, so it remains a leap year. This is one of the most commonly misunderstood edge cases in calendar arithmetic and a frequent source of bugs in date-handling software.
Gregorian vs. Julian Calendar Rules
Before the Gregorian reform of October 15, 1582, most of the Western world used the Julian calendar, whose leap year rule is simpler: every year divisible by 4 is a leap year, with no century exception. Over centuries, this small drift adds up, which is exactly why the Gregorian reform was introduced. Toggle the calendar system in this tool if you're working with historical dates recorded before your region adopted the Gregorian calendar.
Year Range Scanning
Switch to Year Range Scan mode to list every leap year between a start and end year — useful for building calendars, scheduling systems, or verifying date libraries. Scans are capped at 1,000 years per query to keep results fast and the output manageable.
Next and Previous Leap Year
Every single-year check also reports the closest leap year before and after the entered year. This is handy for people born on February 29 (sometimes called "leaplings") who want to know which future years they'll get a real birthday, or for anyone planning events tied to leap day.
Common Uses
Developers use this tool to validate date-handling logic in their own applications. Payroll and finance teams check leap years when calculating daily accrual rates. Event planners and "leapling" birthday celebrants look up upcoming leap years. Students and history enthusiasts use the Julian/Gregorian toggle to understand calendar reform and historical date discrepancies.