Leap Year Checker
Check whether any year is a leap year and contains 366 days.
This tool runs entirely in your browser. Your files are never uploaded to a server.
Common year
2026 has 365 days
What this tool does
Applies the Gregorian calendar's leap-year rule to a year you enter, telling you whether it's a leap year (366 days) or a common year (365 days).
How to use it
Enter a year between 1 and 9999. The result updates immediately as you type; Reset returns to 2026.
Worked example
2026 is not divisible by 4, so it's a common year with 365 days. 2024 is divisible by 4 and not by 100, so it's a leap year with 366 days. 2000 is divisible by 4, by 100, and by 400, so it's also a leap year — while 1900 is divisible by 4 and by 100 but not by 400, making it a common year despite being divisible by 4.
Formula
A year is a leap year if it's divisible by 4, except that century years (divisible by 100) are leap years only when also divisible by 400. This is the Gregorian calendar rule, adopted to keep the calendar aligned with Earth's orbit more precisely than a simple every-4-years rule would.
Why the extra century rule exists
A simple every-4-years leap year would overcorrect — adding roughly one day too many every 128 years, since a solar year is about 365.2422 days rather than exactly 365.25. Removing 3 leap years every 400 years (the century exception, with a partial exception reintroduced for years divisible by 400) brings the average calendar year to 365.2425 days, much closer to the true solar year.
Limitations
This checks the Gregorian calendar rule only — it doesn't apply to the Julian calendar (still used by some religious calendars, where the century exception doesn't exist) or to non-Gregorian calendar systems. Years before the Gregorian calendar's 1582 introduction weren't actually reckoned this way historically, though the calculator still applies the rule proleptically for any year you enter.
Standards and privacy
The rule follows the standard Gregorian calendar definition used internationally (ISO 8601 and civil calendars worldwide). The calculation runs entirely in your browser; the year you enter is never uploaded.
Frequently Asked Questions
- What is the leap year rule?
- A year is a leap year if divisible by 4, except century years (divisible by 100) which are leap years only if also divisible by 400 — so 2000 was a leap year but 1900 was not.
- Why isn't every 4th year simply a leap year?
- A solar year is about 365.2422 days, not exactly 365.25, so a plain every-4-years rule would drift by about a day every 128 years. The century exception corrects for that.
- Does this apply to the Julian calendar?
- No. This checker uses the Gregorian rule with its century exception; the Julian calendar, still used by some religious calendars, treats every 4th year as a leap year without that exception.