DocomintSchaltjahr-Prüfer
Prüfe, ob ein bestimmtes Jahr ein Schaltjahr ist, und warum genau.
What is a Schaltjahr-Prüfer?
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.
How to use the Schaltjahr-Prüfer
- Enter a year
- Press Check
- Whether it's a leap year, and the reasoning behind the result, appear instantly, computed on your device
Formula
Leap year if divisible by 4, EXCEPT century years, which are leap years only if also divisible by 400
The double-exception is what makes 2000 a leap year but 1900 and 2100 not, despite all three being divisible by 4.
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.
Common mistakes
- Assuming any year divisible by 4 is automatically a leap year — century years (divisible by 100) are an exception to that rule.
- Forgetting the exception has its own exception — a century year is a leap year after all if it's also divisible by 400.
- Applying the rule outside a Gregorian-calendar context, where it doesn't necessarily hold.
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.
Will 2100 be a leap year?
No — 2100 is divisible by 100 but not by 400, so it falls under the century-year exception and is not a leap year, the same situation 1900 was in.
How many days does February have in a leap year?
29, instead of the usual 28 — that extra day is what makes a leap year's total 366 days instead of 365.
Is my numbers uploaded anywhere?
No — Schaltjahr-Prüfer runs entirely in your browser using JavaScript/WebAssembly. Your numbers is never sent to a server.