Leap Year Calculator

Leap Year Calculator checks whether a year has 366 days and explains which part of the rule decided it. The rule has three clauses: a year divisible by four is a leap year, unless it is divisible by one hundred, unless it is also divisible by four hundred.

Those exceptions exist because the Earth's orbit is not exactly 365.25 days -- it is about 365.2422. Adding a day every four years overcorrects by roughly 11 minutes a year, so three leap days are removed every four centuries. That is why 1900 was not a leap year and 2000 was, a distinction that caused real software bugs in both years.

Even the corrected calendar drifts, by about one day in 3,200 years. There is no rule for that yet, and there is no particular hurry.

The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.

Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.

• Shows leap year, because, days in the year, next leap year and previous leap year together, so you are not running it twice
• A results table you can read across
• Updates as you type, with no button to press and no page reload
• Copy the result in one click
• Free, no sign-up, and it runs entirely in your browser

The leap year calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

• Checking whether a year is a leap year
• Finding the next or previous leap year
• Explaining the century rule to someone
• Listing upcoming leap years