Leap year calculator

How do I know if a year is a leap year?

In the Gregorian calendar, a year is a leap year if it divides by 4 — except century years, which must also divide by 400. So 2028 is a leap year, 1900 was not, and 2000 was. Among the century years, 1600, 2000, 2400 are leap years and 1700, 1800, 1900, 2100 are not.

When is the next leap year?

2028 — then 2032 and 2036, and every four years until 2096. 2100 will not be a leap year.

Why do we have leap years?

The seasons repeat about every 365.2422 days, which is not a whole number. A calendar of 365 days falls about 0.2422 days short each year, roughly 24 days in a century, so the dates would drift through the seasons. Adding February 29 almost every four years keeps the calendar in step.

How many days are in a leap year?

366 — February has 29 days instead of 28.

What if I was born on February 29?

Your exact birthday comes around only in leap years. The age calculator counts your age on any date.

Why the century exception?

A leap day every fourth year, as in the Julian calendar introduced by Julius Caesar, makes the average year 365.25 days, which is about 0.0078 days longer than the seasonal cycle and adds up to roughly three quarters of a day per century. By the 16th century the spring equinox had slipped from about March 23 to March 11, and in 1582 Pope Gregory XIII reset the calendar and dropped three leap days in every 400 years (the U.S. Naval Observatory’s account). That leaves 97 leap years in a 400-year cycle: 365 × 400 + 97 = 146,097 days, an average of 365.2425 days a year. It stays within about half a minute of the seasonal year, so the calendar would be off by a whole day only after roughly 3,300 years.

Worked example: 1900, 2000, 2024 and 2100

2024 divides by 4 and is not a century year, so it is a leap year. 1900 divides by 4 and by 100 but not by 400, so it was a common year with 365 days. 2000 divides by 400, so it was a leap year. 2100 will be a common year, like 1900, and the next leap year after 2096 is 2104, a gap of eight years.

Common mistakes with leap years

First, using “divisible by 4” alone, which gives the wrong answer for 1900 and 2100. Second, trusting spreadsheets for early dates: Microsoft documents that Excel treats 1900 as a leap year, inherited from Lotus 1-2-3, and the phantom February 29, 1900 makes its WEEKDAY function return wrong values for dates before March 1, 1900. Third, applying the Gregorian rule to dates before 1582, when the calendar did not yet exist; the calculator extends the rule backward, which historians call a proleptic calendar. Fourth, confusing leap years with leap seconds, which are a separate adjustment to clock time. Fifth, forgetting that February 29 shifts everything after it: day 60 is February 29 in a leap year but March 1 in a common year.

Sources: U.S. Naval Observatory, Astronomical Applications FAQ, “Leap years” and “Calendars” (Gregorian rule, 97 leap years per 400, accuracy); Microsoft Support, “Excel incorrectly assumes that the year 1900 is a leap year” (KB 214326). Related: date calculator, age calculator, day of the year calculator, Easter date calculator and calendar.