How this leap year calculator works
A leap year has 366 days instead of the usual 365, with the extra day added to the end of February so that month runs to 29 days. Leap years exist because the Earth takes about 365.2425 days to orbit the Sun — slightly more than a whole number of days. Adding a leap day roughly every four years keeps the calendar dates lined up with the seasons.
The rule is not simply "every four years". A year is a leap year when it is divisible by 4, except for years divisible by 100, unless they are also divisible by 400. Put as one test: a year is a leap year if it is divisible by 4 and (not divisible by 100 or divisible by 400). This calculator applies that test and tells you exactly which part of the rule decided the answer.
Reference: 2000 ✓ (divisible by 400), 1900 ✗ (divisible by 100, not 400), 2024 ✓ (divisible by 4, not 100), 2100 ✗ (divisible by 100, not 400). Calculations use the proleptic Gregorian calendar.
Frequently asked questions
- What is a leap year?
- A leap year is a year with 366 days instead of 365, with an extra day added to February so it has 29 days. The extra day keeps the calendar aligned with the Earth's orbit around the Sun, which takes about 365.2425 days.
- How do you calculate a leap year?
- 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. In short: divisible by 4 AND (not divisible by 100 OR divisible by 400).
- Why is 2000 a leap year but 1900 is not?
- Both are divisible by 100, which would normally skip the leap day. But 2000 is also divisible by 400, so the exception is overridden and it is a leap year. 1900 is not divisible by 400, so it stays a common year with 365 days.
- Is 2024 a leap year?
- Yes. 2024 is divisible by 4 and not divisible by 100, so it is a leap year with 366 days and a 29-day February.
- Why do we have leap years?
- A solar year is roughly 365.2425 days, not a whole number. Without leap years the calendar would drift about a quarter of a day per year and the seasons would slowly move out of step with the dates. A leap day roughly every four years keeps things in sync.
- When is the next leap year?
- Leap years generally occur every four years — 2024, 2028, 2032, 2036 — but century years not divisible by 400, such as 2100, are skipped. Enter a year above to see the next and previous leap years instantly.