Leap Year Calculator

1. Single Year mode: type any year into the field. The calculator instantly tells you whether it is a leap year or not, and lists the next 5 upcoming leap years after it.
2. Year Range mode: enter a start year and an end year to see every leap year that falls within that span. Useful for historical research, scheduling, or academic projects covering multi-decade periods.
3. The result updates as you type, so there is no button to press.

Example: enter 1900. The result is "not a leap year" because 1900 is divisible by 100 but not by 400. Enter 2000 and it is a leap year because it is divisible by 400. Enter 2024 and it is a leap year (divisible by 4, not a century year).

The Gregorian calendar uses a three-part rule to keep the calendar year aligned with the solar year of approximately 365.2422 days:

1. Divisible by 4     → leap year (add one day)
2. Divisible by 100   → NOT a leap year (century exception)
3. Divisible by 400   → IS a leap year (overrides rule 2)

Examples:
2024 → divisible by 4, not by 100 → LEAP YEAR
1900 → divisible by 100, not by 400 → NOT a leap year
2000 → divisible by 400 → LEAP YEAR
2100 → divisible by 100, not by 400 → NOT a leap year

Without leap years, the calendar would drift by about 0.25 days per year. Over 100 years that is 25 days, meaning summer would start in what we now call late June rather than late June relative to the June 21 solstice.

Leap years occur roughly every 4 years but skip 3 times every 400 years (years 100, 200, and 300 of each century). This means there are 97 leap years in every 400-year cycle rather than 100.