Most people know that a leap year comes every four years and adds 29 February. Fewer know that some years divisible by 4 are not leap years at all. The full leap year rules keep our calendar in step with the seasons for centuries. This guide explains them simply and shows how to check any year with our free Leap Year Checker.
The three rules
- A year divisible by 4 is a leap year…
- …except years divisible by 100, which are not…
- …unless they are also divisible by 400, which are.
Examples
| Year | Divisible by 4? | By 100? | By 400? | Leap year? |
|---|---|---|---|---|
| 2024 | Yes | No | – | Yes |
| 2026 | No | – | – | No |
| 2028 | Yes | No | – | Yes |
| 1900 | Yes | Yes | No | No |
| 2000 | Yes | Yes | Yes | Yes |
| 2100 | Yes | Yes | No | No |
Why we need leap years
The Earth takes about 365.2422 days to orbit the Sun. A 365-day calendar would drift by almost a quarter of a day each year, about one day every four years. Over a century, seasons would shift by nearly a month.
Why the century exception?
Adding a day every four years gives an average year of 365.25 days, slightly too long. Skipping three leap days every 400 years (in 1700, 1800 and 1900, but not 2000) brings the average to 365.2425 days, very close to the real value.
The Gregorian calendar
These rules come from the Gregorian calendar, introduced in 1582 to correct the older Julian calendar, which had a leap year every four years without exception. Different countries adopted it at different times.
Upcoming leap years
2028, 2032, 2036, 2040, 2044 and 2048.
Leap years in everyday maths
Leap days affect day counts between dates, age in days and interest calculations. Our Date Difference Calculator and Age Calculator include them automatically. If you or someone you know was born on 29 February, read born on 29 February.
The Islamic calendar is different
The Hijri calendar has its own leap years, adding a day to the last month in 11 years of a 30-year cycle in some calculated versions. See today’s Islamic date with Islamic Date Today (Hijri Converter).
Frequently asked questions
Is 2028 a leap year?
Yes.
Why was 2000 a leap year but 1900 was not?
2000 is divisible by 400; 1900 is divisible by 100 but not by 400.
How many days are in a leap year?
366.