February gets an extra day every four years — mostly. The full rule has three parts: a year is a leap year if it divides evenly by 4, except century years, which must also divide evenly by 400. That is why 2024 and 2028 are leap years, 1900 was not, and 2000 was. The exceptions exist because Earth's orbit takes about 365.2422 days, not 365.25 — without them, the calendar would drift against the seasons.

This calculator checks any year you enter against that rule and returns the verdict. Type 2028 in the Variable A box and the Result confirms a leap year with 366 days; try 2100 and it reports a common year despite dividing by 4, because centuries need the 400 test. It is a handy check when you are validating February dates in spreadsheets, scheduling recurring annual events, or settling the perennial argument about whether 2000 "counted" (it did).

The Gregorian rules used here apply to dates after 1582 in Catholic countries and after 1752 in Britain and its colonies, including America. For historical dates before the switch, the Julian calendar's simpler every-four-years rule applied instead.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Leap Year Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.