Leap Year Calculator

What the leap year calculator does

At its simplest, you give it a year and it tells you whether that year is a leap year, the kind with a 29th of February. It does two more things on top of that. It can list every leap year between two years you choose, and it can find the next leap year after any year. So it covers the quick check and the slightly bigger questions in one place.

How to use it

1. Pick a mode. Check if a year is a leap year, list leap years between two years, or find the next leap year.
2. Enter the year. For the list mode, a second box appears for the end year.

Press Calculate for the answer, or Reset to clear the form.

The leap year rule, and the part people forget

Most people know the first half of the rule: a year is a leap year if it divides evenly by 4. That is true almost all the time, which is why it is easy to stop there. But there is a second part that catches the century years. A year that divides by 100 is only a leap year if it also divides by 400. So 1900 was not a leap year, because it divides by 100 but not by 400, while 2000 was, because it clears both. The tool applies the whole rule, so it gets those century years right where a simple divide-by-4 check would not.

Why the every-four-years rule has an exception

The reason comes down to the length of a year. The Earth takes about 365.2422 days to go around the sun, a little under 365 and a quarter. Adding one day every four years treats that as exactly a quarter, which is slightly too much, and over centuries those tiny overcorrections add up to whole days. Skipping the leap year in three out of every four century years trims that excess back off. So 1700, 1800, and 1900 skipped their leap day, while 2000 kept it, and the calendar stays lined up with the seasons. It is a small piece of bookkeeping doing a surprisingly precise job.

The three things it can do

The check mode is the everyday one, a straight yes or no for a single year. The list mode is for when you want every leap year across a span, handy for filling in a calendar or checking a range at a glance. The next mode answers "when is the leap year coming," stepping forward from the year you give it to the next one. They all use the same rule under the hood, just pointed at different questions.

Where it comes in handy

Mostly for anything that depends on February having 29 days. Checking whether a date like 29 February is valid in a given year. Planning around the date for someone born on the 29th. Sorting out date logic in a spreadsheet or a bit of scheduling. And simple curiosity about whether this year, or a year you have in mind, is one of the leap ones.

Questions people ask

How do I know if a year is a leap year?

It is a leap year if it divides by 4, except for century years, which must also divide by 400. So 2024 is a leap year, 1900 was not, and 2000 was.

Was the year 2000 a leap year?

Yes. It divides by 100, which would normally rule it out, but it also divides by 400, which puts it back in. That is the part of the rule that surprises people.

Can it list all the leap years in a range?

Yes, that is the list mode. Enter a start and end year and it gives you every leap year between them.

Why are there leap years at all?

Because a year is about 365 and a quarter days long, not a round 365. The extra day every four years catches up that quarter, keeping the calendar in step with the seasons.

References

1. U.S. Naval Observatory, Astronomical Applications Department: Leap Years. https://aa.usno.navy.mil/faq/leap_years