Leap Year Calculator
Check whether any year is a leap year, see the next and previous leap years, and count the leap days between two years — including the century exceptions almost everyone forgets.
- Free, no sign-up
- Private — runs in your browser
- Instant results
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The rule in three lines
A year is a leap year if:
- it is divisible by 4 — and
- it is not divisible by 100 — unless
- it is also divisible by 400.
So 2024 is a leap year. 1900 is not, because it is divisible by 100 but not 400. 2000 is, because it clears the 400 test. This third clause is the one almost everyone forgets, and it has caused real software failures — several systems in 2000 and again in 2100-facing calculations have got it wrong.
Why the rule is shaped like that
The Earth takes about 365.2422 days to orbit the sun. A calendar of 365 days would drift by nearly a quarter-day a year, so the Julian calendar added a day every fourth year, giving an average of 365.25.
That still overshot by about 11 minutes annually. Small, but across 1,600 years it accumulated to roughly ten days — enough that the spring equinox had visibly slipped. The Gregorian reform of 1582 corrected the drift and added the century rule to stop it recurring, giving an average year of 365.2425 days. The residual error is about one day in 3,000 years.
How many leap days are in a span?
Over any 400-year cycle there are exactly 97 leap years: 100 years divisible by 4, minus the 4 century years, plus the 1 century year divisible by 400. The calculator counts them exactly rather than approximating, which matters when a span crosses 1900 or 2100.
Leap seconds are a different thing
Leap seconds are occasional one-second adjustments to Coordinated Universal Time, added to keep atomic clocks aligned with the Earth's slightly irregular rotation. They are unpredictable, decided a few months ahead, and unrelated to leap years. They also have no effect on calendar date arithmetic, which is why no age calculator accounts for them.
Frequently asked questions
Is 2100 a leap year?
No. 2100 is divisible by 4 and by 100, but not by 400, so it skips its leap day. The same applied to 1700, 1800 and 1900.
Why was 2000 a leap year when 1900 was not?
Because 2000 is divisible by 400 and 1900 is not. Century years need to clear the 400 test to keep their leap day.
How many leap years are there in 400 years?
Exactly 97. That is 100 years divisible by 4, minus 4 century years, plus the 1 century year divisible by 400.
How do I count leap days between two dates?
Enter a start year and an end year above. The tool counts them exactly, including the century exceptions, rather than dividing by four.
Do leap seconds affect my age?
No. Leap seconds adjust Coordinated Universal Time by one second to track the Earth's rotation. They do not change any calendar date and no age calculator accounts for them.