Leap years explained: why February sometimes has 29 days
By the FreeAllTool team · Updated October 28, 2026
Most years have 365 days, but every few years February gets an extra day. This small adjustment keeps our calendar in line with the seasons, and the rule behind it is a little more detailed than “every four years”.
Why we need leap years
The Earth takes about 365.2422 days to go around the Sun once, measured from one spring equinox to the next. A calendar of exactly 365 days would fall behind by almost a quarter of a day each year. After a century, the seasons would have drifted by about 24 days.
The full rule
In the Gregorian calendar, which is used around the world for civil purposes, a year is a leap year if:
- it is divisible by 4, except
- years divisible by 100 are not leap years, unless
- they are also divisible by 400, in which case they are leap years.
So 2024 and 2028 are leap years. 1900 was not, because it is divisible by 100 but not by 400. 2000 was a leap year because it is divisible by 400. The next century year that will skip being a leap year is 2100.
Why the extra exceptions?
Adding a day every four years gives an average year of 365.25 days, which is slightly too long. Skipping three leap days every 400 years brings the average to 365.2425 days, very close to the true value. The remaining difference adds up to about one day only after several thousand years.
Born on 29 February
People born on 29 February, sometimes called leaplings, have a calendar birthday only once every four years. In other years they usually celebrate on 28 February or 1 March. For legal purposes, many countries treat 1 March as the day they become a year older in non-leap years, although rules can vary. Our age calculator follows the 1 March convention for the next-birthday count.
Leap years and date calculations
When you count days between two dates, any 29 February in between adds one day. That is why a year from 1 March 2027 to 1 March 2028 has 366 days.