Leap Year Calculator

Frequently Asked Questions

Is 2100 a leap year?

No. 2100 is divisible by 4 and by 100 but not by 400, so it is a common year with 365 days.

When is the next century-year leap year after 2000?

The year 2400. The years 2100, 2200, and 2300 are all skipped.

How rare is a February 29 birthday?

Roughly 1 in 1,461, since leap days occur about once every four years. Someone born on February 29 has an exact calendar birthday only in leap years.

Why is the century rule (skip leap year on multiples of 100) necessary?

Without it, adding a day every 4 years gives an average year of 365.25 days, which is about 11 minutes longer than the true solar year. Skipping century years removes most of that excess. Adding them back on 400-year multiples fine-tunes the average to 365.2425 days.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Enter any year from 1 onward. The tool applies the Gregorian leap-year rule to tell you whether that year has 366 days or 365 days, then scans outward to report the previous and next leap years for context. Because the rule is purely arithmetic, the result is exact for any year you enter. A leap year is one where February has 29 days instead of 28.

The Gregorian Leap Year Rule

A year is a leap year if it satisfies either of these conditions:
(a) it is divisible by 4 and not by 100, or
(b) it is divisible by 400.
In code: (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0. Common years (1900, 2100, 2200, 2300) are divisible by 4 and by 100 but not by 400, so they are not leap years. The years 2000 and 2400 are divisible by 400, so they are leap years despite the century-year rule.

Worked Example

1900: divisible by 4 and by 100 but not by 400 → not a leap year (365 days). 2000: divisible by 400 → leap year (366 days). 2024: divisible by 4 and not by 100 → leap year (366 days). 2025: not divisible by 4 → not a leap year (365 days). 2100: divisible by 4 and by 100 but not by 400 → not a leap year.

Why Leap Years Exist

Earth takes approximately 365.2422 days to orbit the Sun. Adding a day every four years gives an average of 365.25 days, but that slightly overcorrects. Skipping the leap day on century years brings the average to 365.24, slightly under. Adding it back on 400-year multiples raises the average to 365.2425, within about 26 seconds of the true solar year. Without leap years, the calendar would drift about 24 days every 100 years, eventually shifting summer months into winter.

Common Mistakes and FAQ

Assuming every year divisible by 4 is a leap year: century years like 1900 are the exception.

Forgetting the 400-year override: it makes 2000 a leap year but 2100 will not be.

February 29 birthdays: these occur roughly once every 1,461 days (about 1 in 1,461 chance), since the extra day comes every four years.

Julian vs. Gregorian: the older Julian calendar used a simpler "every 4 years" rule, which overestimates by about 11 minutes per year.

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