How Many Hours Are in a Day?

A day has 24 hours, which is 1,440 minutes or 86,400 seconds. That's the everyday answer, but there are a few interesting exceptions.

Why 24?

We inherited the number from ancient Egypt, where the day and the night were each divided into 12 parts. Those hours varied in length with the seasons, so a summer daytime hour was longer than a winter one. Equal-length hours became standard with mechanical clocks in medieval Europe.

Days that aren't 24 hours

In places that use daylight saving time, the calendar has two unusual days each year:

• Spring: clocks jump forward and that day has 23 hours.
• Autumn: clocks go back and that day has 25 hours.

It's the reason some software errors appear twice a year, and why our date calculator counts in calendar days rather than multiplying by 24.

Solar days and sidereal days

A solar day is the time between one noon and the next. It averages 24 hours but changes slightly through the year, by up to about half a minute, because Earth's orbit is an ellipse and its axis is tilted.

A sidereal day is how long Earth takes to spin once relative to the distant stars: about 23 hours, 56 minutes and 4 seconds. The difference exists because Earth also moves along its orbit, so it needs to turn a little further to bring the Sun back to the same position.

Are days getting longer?

Yes, very slowly. The Moon's tides act as a brake on Earth's rotation, and days have been lengthening over millions of years by tiny amounts. Short-term rotation also varies, which is why official time has occasionally been adjusted with leap seconds. Read more in what UTC time is.

See how today is going on the 24-hour rail on our home page.