Online Tool Store
Leap Year Checker
Check whether a year is a leap year, with an explanation of the rule and a list of upcoming leap years. Runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
—
—
Next leap years
How it works
- Enter any year.
- See whether it's a leap year and why, plus the next five leap years.
The rule
Leap year if: (year % 4 == 0 AND year % 100 != 0) OR (year % 400 == 0)
FAQ
What is the leap year rule?
A year is a leap year if it's divisible by 4, except century years (divisible by 100), which are only leap years if also divisible by 400. This is the Gregorian calendar rule — it's why 2000 was a leap year but 1900 and 2100 are not.
Why does the rule need the century exception?
A year divisible by 4 alone would add one leap day too many every few centuries — the calendar would drift out of sync with the seasons over time. The 100/400 exception keeps the average calendar year closer to the actual astronomical year length.
Does this work for years before the Gregorian calendar existed?
The calculation assumes the modern Gregorian rule throughout — many countries didn't adopt it until centuries after 1582, so historical dates in the Julian calendar may follow a different (simpler, divisible-by-4-only) leap year rule.
Is my data uploaded anywhere?
No — the calculation happens entirely in your browser. Nothing is sent to a server.
How we compare
| Feature | Online Tool Store | Mental math | Calendar app lookup |
|---|---|---|---|
| Explains the "why" for century years | ✓ | Easy to get 1900/2000 wrong | No explanation given |
| Lists upcoming leap years | ✓ | Manual calculation | Scroll through months |
For quickly checking a tricky century year — the exact case most people get wrong — this is faster and more reliable than doing the division in your head.