Leap years explained
Learn the Gregorian leap-year rule and why February sometimes has 29 days.
The Gregorian rule
A year divisible by 4 is usually a leap year. A year divisible by 100 is not, unless it is also divisible by 400. Therefore 2000 was a leap year, while 1900 was not.
Why leap days exist
Earth's orbit is not exactly 365 days. Leap-day rules keep calendar seasons from drifting steadily away from the astronomical year.
Effects across the calendar
Dates after February move one ordinal position later in a leap year. Annual rates and daily averages should use 366 as the total when appropriate.
Related year guides
- What day number is today? — Understand ordinal dates and find today's position in the year.
- How many weeks are in a year? — Why a year is about 52 weeks and why some ISO years have week 53.
- How to calculate year progress — A precise formula for calculating year progress in code or a spreadsheet.