Why do century years like 1900 break the simple rule?

The extra divisible-by-100-but-not-400 exception keeps the calendar aligned with the solar year. Without it, the calendar would drift by about three days every 400 years.

Is anything sent to a server?

No. The check is pure local arithmetic in your browser — no data is uploaded or stored.

Leap Year Checker

Q: What makes a year a leap year?

A year is a leap year if it is divisible by 4, except for century years, which must also be divisible by 400. So 2000 was a leap year but 1900 was not. This checker applies that rule and explains which part of it decided the result.

Check if any year is a leap year and see why.

Is it a leap year?

A leap year has 366 days instead of 365, adding 29 February to keep the calendar aligned with the solar year. The rule is: a year is a leap year if it is divisible by 4, except century years, which must also be divisible by 400 — so 2000 was a leap year but 1900 was not.

Type any year and this checker gives an instant verdict, a plain-English explanation of which rule applied, and the next leap year — all computed in your browser.