Leap Year Calculator

Leap year rules and checks

A leap year adds February 29 to keep the civil calendar aligned with the solar year. In the Gregorian calendar, most years divisible by 4 are leap years, but century years are an exception unless they are also divisible by 400. This is why 2000 was a leap year but 1900 and 2100 are not.

How to use it accurately

1. Check divisibility by 400: If true, the year is a leap year.
2. Otherwise check divisibility by 100: If true, it is not a leap year.
3. Otherwise check divisibility by 4: If true, it is a leap year; otherwise it is a common year.
4. Use the result for date arithmetic: A leap year has 366 days and shifts ordinal dates after February by one.

Worked examples

How to interpret the result

Leap Year Calculator should be interpreted together with the inputs and the convention used to produce the answer. The core relationship on this page is “leap = divisible by 400 OR (divisible by 4 AND not divisible by 100)”. The century exception prevents the calendar from gaining too many leap days over long periods while the 400-year rule restores the needed correction. In practical use, that means the displayed number is not just an isolated output: it represents a result under the assumptions stated on this page. Date calculations depend on calendar conventions such as inclusive versus exclusive endpoints, month length, leap years, weekday rules, and sometimes holidays. A result is most useful when the counting convention is recorded with it.

A useful sanity check is to compare your result with a nearby worked example. For “Year 2000”, the inputs divisible by 400 produce Leap year. Although it is a century year, divisibility by 400 makes it leap. A second boundary check is “Year 2100”: divisible by 100 but not 400 produces Not a leap year. The ordinary divisible-by-4 rule is overridden for this century year. If your result moves in the opposite direction from these examples when you change a similar input, recheck units, signs, endpoints, rounding, or any special option before copying the answer elsewhere.

How to verify the calculation

1. Re-read the original inputs before checking the result. A correct formula cannot compensate for an incorrect date, time, rate, duration, amount, or sign entered at the start.
2. Recalculate the central relationship once by hand or with a simple independent method: leap = divisible by 400 OR (divisible by 4 AND not divisible by 100). The purpose is not to reproduce every display detail, but to confirm that the answer is in the right range and direction.
3. Check the output units and formatting. The reference table “Century examples” is a second quick check when your inputs fall near one of the listed values.
4. Test the edge case that is most similar to your situation. One important warning on this calculator is: Not every year divisible by 4 is a leap year; century exceptions matter.
5. Before using the result for a consequential decision, read the assumptions section. The calculator uses Gregorian leap-year rules, including for years where a historical region may have used another calendar. For historical scholarship, use the calendar system actually in force at the place and time.

When this calculator is useful

• Checking whether February has 28 or 29 days.
• Validating date ranges across February.
• Explaining day-of-year differences after February.
• Planning recurring events around February 29.

Century examples

Common mistakes and edge cases

• Not every year divisible by 4 is a leap year; century exceptions matter.
• The Julian calendar uses a different historical rule.
• A leap year has 366 days, but a calendar year interval can still depend on endpoints.
• Legal treatment of February 29 birthdays can be jurisdiction-specific.

Assumptions and limitations

The calculator uses Gregorian leap-year rules, including for years where a historical region may have used another calendar. For historical scholarship, use the calendar system actually in force at the place and time.

Frequently asked questions