DocomintLeap Year Checker
Check whether a given year is a leap year, and see exactly why.
The leap year rule has an exception people often miss: divisible by 4 is a leap year, *unless* it's also divisible by 100, in which case it isn't — *unless* it's also divisible by 400, in which case it is after all. That's why 2000 was a leap year but 1900 wasn't, despite both being divisible by 4 and by 100.
Example: 2026 is not a leap year (not divisible by 4), while 2000 was a leap year (divisible by 400, overriding the divisible-by-100 exception) and 1900 was not (divisible by 100 but not by 400) — three different outcomes from what looks like the same rule at a glance.
How it works
- Enter a year
- Press Check
- Whether it's a leap year, and the reasoning behind the result, appear instantly, computed on your device
Frequently asked questions
Why was 1900 not a leap year but 2000 was?
Both are divisible by 100, which normally disqualifies a year from being a leap year even if it's divisible by 4 — but there's a further exception for years divisible by 400. 2000 is divisible by 400 (so it was a leap year); 1900 is not (so it wasn't).
How often do leap years occur?
Roughly every 4 years, with the exception that century years not divisible by 400 are skipped — this works out to 97 leap years every 400 years rather than a flat 100, which is what keeps the calendar aligned with Earth's actual orbital period over long spans of time.
Is my numbers uploaded anywhere?
No — Leap Year Checker runs entirely in your browser using JavaScript/WebAssembly. Your numbers is never sent to a server.