How many hours are in a day?

There are 24 hours in a day.

In ordinary civil timekeeping, a day runs from midnight to midnight and is divided into 24 hours. Each hour contains 60 minutes, and each minute contains 60 seconds, giving the conventional day a total of 86,400 seconds. This system makes clocks and calendars practical for everyday life, even though Earth’s rotation is not perfectly uniform.

The modern civil day usually begins at 00:00, or midnight, and ends at 24:00, which is the same instant as 00:00 on the following date. The 12-hour clock expresses the same cycle as a.m. and p.m.; the 24-hour clock labels the hours from 00 through 23.

There is an astronomical wrinkle: a solar day varies slightly because Earth both rotates on its axis and moves around the Sun. A sidereal day, measured against distant stars, is about 23 hours, 56 minutes, and 4 seconds. For everyday schedules, however, “a day” conventionally means 24 hours. Leap seconds and calendar adjustments help keep civil time aligned with Earth’s changing rotation.