Leap Year Calculator

Check if any year is a leap year and see upcoming leap years.

Before 1582, the Julian calendar was in use — all years divisible by 4 were leap years (no skipping). The ÷100/÷400 rules shown here apply only to the Gregorian calendar (1582–present).

Leap Year Rules

The Gregorian calendar, introduced by Pope Gregory XIII in 1582, uses three rules to determine leap years:

  1. If the year is divisible by 4, it might be a leap year
  2. If also divisible by 100, it is not a leap year
  3. If also divisible by 400, it is a leap year

Examples: 2000 was a leap year (÷400), 1900 was skipped (÷100 but not ÷400), 2024 is a leap year (÷4, not ÷100).

Why Leap Years Exist

Earth's orbital period is approximately 365.2422 days. Without leap years, the calendar would drift ~1 day every 4 years, eventually shifting seasons. The Gregorian correction (skipping 3 leap years every 400 years) keeps the average year length at 365.2425 days — accurate to within 1 day per 3,236 years. Over very long timescales, further calendar reform may be needed, but the ÷4/÷100/÷400 rules remain the accepted standard for the foreseeable future.

Programming Considerations

Feb 29 validation

Date inputs must reject Feb 29 in non-leap years

Day-of-year offset

Mar 1+ day numbers shift by 1 in leap years

Age calculation

People born Feb 29 — when is their birthday?

Recurring events

Annual events on Feb 29 need fallback logic

Sources