Leap Year Checker
Is a year a leap year?
About This Calculator
A leap year has 366 days instead of the usual 365, with February having 29 days instead of 28. Leap years correct the drift between the calendar year and the astronomical year (365.2422 days). The Gregorian calendar rule: divisible by 4, except century years must be divisible by 400.
Formula
Leap year if: (year % 4 == 0) AND (year % 100 != 0 OR year % 400 == 0)
Century years (1700, 1800, 1900) are NOT leap years
400-year cycle years (1600, 2000, 2400) ARE leap years
Average year: 365 + 97/400 = 365.2425 days (very close to 365.2422)
Example Calculation
Is 2100 a leap year?
- 2100 is divisible by 4: yes (2100/4=525)
- 2100 is divisible by 100: yes (2100/100=21)
- 2100 is divisible by 400: no (2100/400=5.25)
- Rule: divisible by 100 but NOT by 400 → NOT a leap year
2100 is not a leap year
Leap Year Rules Summary
| Divisible by 4? | Divisible by 100? | Divisible by 400? | Leap Year? | Example |
|---|---|---|---|---|
| No | — | — | No | 2019 |
| Yes | No | — | Yes | 2024 |
| Yes | Yes | No | No | 1900, 2100 |
| Yes | Yes | Yes | Yes | 1600, 2000, 2400 |
Frequently Asked Questions
Why do we have leap years?
The Earth takes approximately 365.2422 days to orbit the Sun. Without leap years, the calendar would drift about 1 day every 4 years, causing seasons to shift over centuries. The Gregorian leap year rule keeps the calendar aligned with the solar year to within 1 day per ~3,236 years.
Were there always leap years?
The Julian calendar (46 BC) added a leap day every 4 years without exception, causing a drift of ~11 minutes/year. By 1582, it had drifted 10 days. Pope Gregory XIII introduced the Gregorian calendar skipping century leap years (except every 400 years), correcting the accumulated error.
What happens if you're born on February 29?
Leap day babies (leaplings) legally celebrate their birthday on February 28 or March 1 in non-leap years, depending on jurisdiction. They only have an exact calendar birthday every 4 years (with occasional exceptions at century years). In 2024, there were about 5 million leaplings worldwide.
Will any future calendar reform eliminate leap years?
The proposed Revised Julian Calendar and the perpetual calendar concepts address this. The World Calendar and International Fixed Calendar proposals would use a more regular structure, but no international calendar reform has gained adoption. The Gregorian calendar is expected to remain standard.