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Gregorian Calendar

Also called: civil calendar, Western calendar

Definition: The Gregorian calendar is the civil calendar in worldwide use. Its 400-year cycle has 146,097 days, an average year of 365.2425 days, kept by the leap-year rule.

Updated November 4, 2026 · How we check facts

The Gregorian calendar is the calendar used for civil dates in most of the world. Its rule for leap years makes the calendar repeat every 400 years. The U.S. Naval Observatory's calendar chapter gives that cycle as 146,097 days, or 365.2425 days per year, close to the length of the tropical year.

Formula

mean year = (303 × 365 + 97 × 366) ÷ 400 = 365.2425 days

A 400-year cycle has 97 leap years and 303 common years.

Example

The cycle length is 303 × 365 + 97 × 366 = 146,097 days. Since 146,097 = 20,871 × 7, a whole number of weeks, weekdays repeat exactly every 400 years. The mean year is 365.2425 ÷ 7 = 52.1775 weeks (how many weeks in a year).

Calculate it

Free toolDate Duration CalculatorDays between two dates, or a date plus N days.

Guides that explain it

Related terms

Leap YearA leap year has 366 days instead of 365. In the Gregorian calendar a year is a leap year if divisible by 4, except centurial years that are not divisible by 400.Tropical YearThe tropical year is the time between successive spring equinoxes, about 365.2422 days (about 31,556,930 seconds). It is the natural year the Gregorian calendar approximates.Hour (h)The hour is a unit of time equal to exactly 60 minutes or 3,600 seconds, and one 24th of a day. It is accepted for use with the SI but is not an SI unit itself.Leap SecondA leap second is a one-second adjustment added to Coordinated Universal Time (UTC) so it stays within 0.9 s of Earth-rotation time, giving a day of 86,401 seconds.Minute (min)The minute is a unit of time equal to exactly 60 seconds, one 60th of an hour. It is accepted for use with the SI, and a day has 1,440 of them.Second (s)The second is the SI base unit of time, defined from the caesium-133 transition frequency of 9,192,631,770 Hz; there are exactly 60 seconds in a minute and 86,400 in a day.Sidereal DayA sidereal day is one rotation of the Earth relative to the stars: about 86,164.09 seconds, or 23 h 56 min 4.09 s, roughly 3 min 56 s shorter than a 24-hour solar day.

Sources

  1. U.S. Naval Observatory — Leap Years