Leap Year Calculator

A leap year has 366 days instead of 365 — February gains an extra day (the 29th). This extra day exists to keep the calendar aligned with Earth's orbit around the Sun, which is not a whole number of days.

The rules (Gregorian calendar)

A year is a leap year if:

1. Divisible by 4 → yes, a leap year
2. But if it is also divisible by 100 → not a leap year (a century year rule)
3. Unless it is also divisible by 400 → then yes, a leap year after all

So 2000 was a leap year, but 1900 and 2100 are not.

Why the exceptions?

The problem is that a tropical (solar) year is about 365.2422 days — not exactly 365.25. A naive rule of adding one day every 4 years (the Julian approach) assumes 365.25, which overcorrects by roughly 11 minutes a year, or about 3 days every 400 years. That drift pushed the calendar out of step with the seasons over centuries — a major concern for church and agriculture alike. The century rule (skipping leap years divisible by 100) compensates, and the 400-year exception fine-tunes it. The result: over any 400-year cycle there are exactly 97 leap years, keeping the calendar accurate to within a day every 3,300 years or so.

What the calculator shows

• Is it a leap year? — a simple yes or no from the rule above
• Days in the year — 365 or 366
• February days — 28 or 29, the concrete effect of a leap year
• Next leap year — the next eligible year after the one you enter, handy for planning ahead

Why leap years matter in everyday life

Beyond trivia, leap years and days directly affect:

• Payroll and billing — a leap February adds a paid day or shifts prorated amounts
• Interest and tenure calculations — day-count conventions depend on whether a year has 365 or 366 days
• Age and contract dates — birthdays and deadlines that fall on 29 February need special handling in non-leap years
• Software and scheduling — recurring events and date math must not break on the 29th

Fun facts

• 29 February is called a "leap day"; people born on it — "leaplings" — technically celebrate their birthday only every four years, and most celebrate on 28 February or 1 March in other years.
• The chance of being born on 29 February is about 1 in 1,461 (roughly one day across a four-year span of 1,461 days).
• There are 97 leap years in every 400-year Gregorian cycle, which is why the average Gregorian year is about 365.2425 days.

Note: historical dates before 1582, or in regions that adopted the Gregorian calendar late (such as Britain in 1752), may follow the older Julian leap-year rules — relevant if you are checking historical documents.

Historical note

The leap-year system was refined by the Gregorian calendar reform of 1582, introduced by Pope Gregory XIII to correct a drift that had accumulated for centuries under the simpler Julian system — the reason the century and 400-year exceptions exist at all.