Leap Year Checker
Type a year and find out whether it has a 29 February — and, more usefully, which rule decided it. Nothing is uploaded.
| Year | Leap? | Because |
|---|
| Year | Divides by 4 | Divides by 100 | Divides by 400 | Leap? |
|---|
Three of these eight are leap years. A check that only tests for division by four gets five of them wrong.
How to use it
- Type any year.
- The answer says yes or no, and the line underneath says which of the three rules produced it.
- The tables show the years either side and the century years, which are where the rule stops being obvious.
The rule, in the order it is applied
- Does it divide by 4? If not, it is an ordinary year and there is nothing more to check.
- Does it divide by 100? If not, it is a leap year. This covers almost every leap year you will ever meet.
- Does it divide by 400? If it does, it is a leap year after all. If it does not, the leap day is skipped.
So 2024 is a leap year at step two. 1900 fails at step three and is not. 2000 passes step three and is. That last pair is the whole reason the rule exists in the form it does.
Why a leap day is needed at all
A year is not a whole number of days. The Earth takes about 365.24219 days to get back round the Sun, so a 365 day calendar loses very nearly a quarter of a day each year. Left alone, the seasons drift: after a century the calendar is out by around 24 days, and midsummer starts sliding towards spring.
Adding a day every four years overcorrects slightly, to 365.25. Skipping three leap days every four centuries brings the average to 365.2425, which is within half a minute a year of the truth. That is close enough to hold the date of the equinox steady for thousands of years, which is exactly what the Gregorian reform of 1582 was for.
The years this page is honest about
This checker applies the modern rule to every year you give it, including years long before the rule existed. That is a deliberate and standard choice — it is called the proleptic Gregorian calendar, and it is what software and historians use so dates can be compared at all.
It is not what people living then would have recognised. The Gregorian calendar began in October 1582 and was adopted country by country over the next three and a half centuries; Britain and its colonies only changed in 1752, and skipped eleven days to catch up. For a year before your country's changeover, treat the answer here as the modern arithmetic rather than a historical fact.
Frequently asked questions
Why was 1900 not a leap year when 2000 was?
A year divisible by 100 skips its leap day, unless it is also divisible by 400. 1900 divides by 100 and not by 400, so it was an ordinary year of 365 days. 2000 divides by 400, so it kept its 29 February. This is the rule most calendar software got wrong in 1999, because a great deal of it tested only for divisibility by four.
What is the leap year rule in full?
A year is a leap year if it divides by 4, except that it is not if it divides by 100, except that it is again if it divides by 400. That gives 97 leap years every 400 years rather than 100, making the average calendar year 365.2425 days long against the true 365.24219.
When is the next leap year?
Leap years run every four years with the century exception, so after 2024 come 2028, 2032, 2036 and 2040. The next skipped one is 2100, which divides by 100 but not by 400 and so will have no 29 February. Nobody alive has seen a skipped century year, because the last was 1900.
Is anything I type sent anywhere?
No. The arithmetic happens in JavaScript inside your own browser. Nothing is uploaded and nothing is saved - close the tab and it is gone.