Leap Year Rules: Why 2100 Isn't One and 2000 Was
October 21, 2026
A year is a leap year if it is divisible by 4, except for years divisible by 100, unless they are also divisible by 400. So 2024 is a leap year, 1900 and 2100 are not, and 2000 was. The rule gives the Gregorian calendar an average year length of 365.2425 days, very close to the actual solar year of about 365.2422 days.
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Open the Date Calculator →Why We Need Leap Years
Earth takes about 365.2422 days to orbit the Sun. A calendar with exactly 365 days drifts by about a quarter day each year, which adds up to roughly one day every four years and about 25 days in a century. Without correction, seasons would slowly shift against the calendar dates.
Adding one day every four years averages out to 365.25 days, which is slightly too long. Skipping three leap years every 400 years brings the average down to 365.2425.
The Rule Step by Step
Test a year in this order. First, if it is divisible by 400, it is a leap year. Otherwise, if it is divisible by 100, it is not. Otherwise, if it is divisible by 4, it is a leap year. Otherwise it is not.
Over every 400 years there are 97 leap years: 100 years divisible by 4, minus the 3 century years that are not divisible by 400.
| Year | Divisible by | Leap year? |
|---|---|---|
| 2024 | 4 | Yes |
| 1900 | 4 and 100, not 400 | No |
| 2000 | 4, 100, and 400 | Yes |
| 2100 | 4 and 100, not 400 | No |
| 2023 | none | No |
Effects on Date Calculations
The number of days between two dates changes by one if a February 29 falls between them. This matters for interest on day-count conventions, contract deadlines, and exact age. A person born on February 29 may celebrate on February 28 or March 1 in common years depending on local law.
A year has 366 days in a leap year, so annual rates and 365-day divisors can be off by 0.27% in leap years. Check the convention a contract or lender uses.
Frequently Asked Questions
Is 2100 a leap year?
No. It is divisible by 100 but not by 400.
Why was 2000 a leap year?
It is divisible by 400, which overrides the century exception.
How many leap years are there in 400 years?
97, giving 146,097 days in total, an average of 365.2425 days per year.
When is the next leap year after 2028?
2032. Ignoring century exceptions, leap years fall every four years.