Check a single year, or list every leap year in a range.
Is it a leap year?
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How this works
A year is a leap year if divisible by 4,
except century years (divisible by 100), which must also be divisible by 400.
2000 was a leap year (divisible by 400); 1900 and 2100 are not (divisible by 100 but not 400).
A leap year adds 29 February, keeping the calendar year in step with Earth's orbit, which is close to but not exactly 365 days.
Leap years recur on a repeating but uneven pattern because of the century-year exception — they are not always exactly 4 years apart across a century boundary.
Why is 1900 not a leap year but 2000 was?
Both are divisible by 4 and by 100, which normally rules a year out. But years divisible by 400 are leap years anyway — 2000 ÷ 400 = 5 exactly, while 1900 ÷ 400 does not divide evenly, so 1900 was not a leap year.
How often do leap years happen?
Every 4 years, with the exception that century years not divisible by 400 are skipped — so 97 times every 400 years rather than exactly 100.
Why do we need leap years at all?
A year (one orbit of the sun) takes about 365.2422 days, not exactly 365. Without leap years, the calendar would slowly drift out of sync with the seasons — about a day every 4 years.