Leap Year Calculator
A leap year adds a 29th day to February to keep the calendar aligned with Earth's orbit, which takes about 365.2422 days. The Gregorian rule is precise: a year is a leap year if it is divisible by 4, but century years are leap years only if they are also divisible by 400. Enter any year and this calculator tells you whether it is a leap year, how many days it has, how many days February has, and the next leap year, all from the exact arithmetic rule with no estimation.
Gregorian leap-year rule
Leap year if (year mod 4 == 0)
and not (year mod 100 == 0)
unless (year mod 400 == 0)
Days in year = 366 if leap, else 365
Days in February = 29 if leap, else 28
In short: divisible by 4 is a leap year, divisible by 100 is not, but divisible by 400 is again. The next leap year is found by scanning forward from the entered year.
Calendar context
- The mean tropical year is about 365.2422 days, which the leap rule approximates very closely.
- The years 2000 and 2400 are leap years; 1900, 2100, 2200, and 2300 are not.
- The Gregorian calendar was introduced in 1582 to correct drift in the older Julian calendar.
- February 29 is sometimes called leap day; people born on it are sometimes called leaplings.
- The 400-year Gregorian cycle contains exactly 97 leap years and 146,097 days.
Leap year calculator: frequently asked questions
What is the rule for a leap year?
Under the Gregorian calendar a year is a leap year if it is divisible by 4, except that years divisible by 100 are not leap years unless they are also divisible by 400. So 2024 and 2000 are leap years, but 1900 and 2100 are not. This calculator applies that exact rule.
Why do we skip leap years in some century years?
The solar year is about 365.2422 days, slightly less than 365.25. Adding a leap day every 4 years over-corrects by about 3 days every 400 years. Dropping the leap day in 3 out of every 4 century years (those not divisible by 400) brings the calendar back in line with the seasons.
Was 2000 a leap year?
Yes. The year 2000 is divisible by 100, which would normally exclude it, but it is also divisible by 400, so the exception to the exception applies and it is a leap year. The next century leap year is 2400; 2100, 2200, and 2300 are common years.
How many days are in a leap year?
A leap year has 366 days because February has 29 days instead of 28. A common (non-leap) year has 365 days. The extra day keeps the calendar aligned with Earth's orbit around the Sun over the long term.
Does this work for years before 1582?
The Gregorian calendar was introduced in 1582, replacing the Julian calendar, which treated every year divisible by 4 as a leap year with no century exception. This calculator applies the Gregorian (proleptic) rule to any year you enter, which is the standard convention for modern dates.
Official sources
- U.S. Naval Observatory: Leap years explained.
- NIST: Calendars and timekeeping.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.