Leap Year Calculator

What is a Leap Year?

A leap year is a calendar year that contains an additional day (February 29th) added to keep the calendar year synchronized with the astronomical year. The Earth takes approximately 365.25 days to orbit the Sun, so we add an extra day every four years to account for the quarter-day difference.

The Rules

• A year is a leap year if it is divisible by 4
• EXCEPT if it is divisible by 100 (then it's not a leap year)
• UNLESS it is also divisible by 400 (then it is a leap year)

Why the 100 and 400 Rules?

The Earth's orbit is actually 365.2422 days, not exactly 365.25. Adding a leap day every 4 years slightly overcorrects. The 100-year exception removes 3 leap days every 400 years, making the average year 365.2425 days—much closer to the actual orbital period.

Historical Context

The leap year system was introduced by Julius Caesar in 45 BCE (Julian calendar) with just the 4-year rule. Pope Gregory XIII refined it in 1582 (Gregorian calendar) by adding the 100 and 400-year exceptions, creating the system we use today.

Fun Facts

• People born on February 29th are called "leaplings" or "leapers"
• The year 2000 was a leap year, but 1900 and 2100 are not
• Leap years occur every 4 years, except 3 times every 400 years
• There's a 1 in 1,461 chance of being born on leap day
• In Ireland, tradition says women can propose to men on leap day
• The next non-leap century year is 2100

Leap Year Frequency

In a 400-year period, there are 97 leap years (not 100). This gives an average year length of 365.2425 days, which is accurate to within 1 day every 3,236 years.