Leap Year Calculator
Check whether any year is a leap year, and how many days it has.
Comment ça marche
This calculator checks whether any given year is a leap year (366 days, with an extra day added to February) using the standard Gregorian calendar rule: a year is a leap year if it's divisible by 4, except century years, which must be divisible by 400.
The reason leap years exist at all comes down to astronomy: Earth actually takes about 365.2425 days to orbit the Sun, not a clean 365. Without correction, a calendar that always used exactly 365 days would slowly drift out of sync with the seasons — over just a few centuries, the calendar date of the spring equinox would shift noticeably.
Adding a leap day roughly every 4 years compensates for that extra quarter-day per year, keeping the calendar closely aligned with the astronomical year. But 4 years isn't a perfectly clean fix either, since 0.2425 is slightly less than exactly 0.25 — this small residual error is what the century-year exception corrects for.
The century-year rule works like this: a year divisible by 100 is NOT a leap year, unless it's also divisible by 400. So 1900 (divisible by 100 but not 400) was not a leap year, while 2000 (divisible by both 100 and 400) was. This double-exception fine-tunes the calendar's average year length to almost exactly match the real astronomical year.
This system — called the Gregorian calendar — was introduced in 1582, replacing the older Julian calendar, which used a simpler "every 4 years" rule without the century-year exception, and had drifted noticeably out of alignment with the seasons over the many centuries it had been in use.
Leap years matter beyond simple curiosity: software that calculates ages, durations, or schedules needs to correctly account for February having 29 days in a leap year — a surprisingly common source of off-by-one bugs in date-handling code, and part of why this specific rule (rather than a rough approximation) is worth checking explicitly.
People born on February 29th (sometimes called "leaplings") face a quirky practical question every non-leap year: which date to treat as their birthday. Most commonly, this is resolved as either February 28th or March 1st, depending on personal preference or, in some legal and administrative contexts, jurisdiction-specific convention.
Why the century-year exception?
It corrects for the fact that a year is slightly less than 365.25 days — the extra rule keeps the calendar aligned with Earth's orbit over centuries.
Was the year 2000 a leap year?
Yes — 2000 is divisible by both 100 and 400, satisfying the century-year exception, so it was a leap year despite being a century year.
Was the year 1900 a leap year?
No — 1900 is divisible by 100 but not by 400, so the century-year exception applies and it was NOT a leap year.
How many days does February have in a leap year?
29 days, one more than the usual 28 — this extra day is what makes a leap year 366 days total instead of 365.
Why isn't a year exactly 365.25 days?
The actual figure is closer to 365.2425 days — the small difference from 365.25 is why the simple "every 4 years" rule needs the additional century-year correction to stay accurate over long periods.
What calendar system introduced the leap year rules used today?
The Gregorian calendar, introduced in 1582, replacing the older Julian calendar's simpler every-4-years rule, which lacked the century-year exception and had drifted out of alignment with the seasons.
How do I quickly check if a year is a leap year?
Check if it's divisible by 4; if it's also a century year (ending in 00), it must additionally be divisible by 400 to qualify.
Why do leap years matter for software and date calculations?
Code that calculates ages, durations, or schedules needs to correctly account for February's extra day in leap years — a common source of subtle bugs if the rule isn't applied correctly.
What do people born on February 29th do for a birthday in non-leap years?
Most commonly celebrate on either February 28th or March 1st, since their exact birth date doesn't exist in non-leap years — the choice often comes down to personal preference or local convention.
What's the next century year that will be a leap year?
2400 — since it's divisible by 400. The centuries in between (2100, 2200, 2300) will not be leap years, as none are divisible by 400.