An hour is a unit of time based on the second.

What is an hour

For everyday use, an hour is defined as 60 minutes, or 3600 seconds.

Definition of an hour

The definition of an hour has changed over time. The modern definition of an hour is based on the atomic second, which is precisely defined as the fixed numerical value of the caesium frequency, which is 9,192,631,770 when expressed in Hz, or s-1.

Technically, an hour is defined scientifically as 3599 to 3601 seconds, because in some cases a leap second, +1 or -1 is necessary to ensure that the measured time is exact.

Historically, the second was defined as of the mean solar day, which is based on the position of the Sun in the sky.

Like most SI units, there are numerous measurements that are derived from an hour. Some examples include:

Convert hours

Below are some conversions from hours to other units of time.

Hours to minutes

There are 60 minutes in 1 hour. To convert from hours to minutes, divide by 60. To convert from minutes to hours, multiply by 60.

Hours to days

There are 24 hours in 1 day. To convert hours to days, divide by 24. To convert days to hours, multiply by 24.

How many hours in a week

There are 168 hours in 1 week. To convert hours to weeks, divide by 168. To convert weeks to hours, multiply by 168.

How many hours in a month

There are 730 hours in 1 month. To convert from hours to months, divide by 730. To convert from months to hours, multiply by 730.

Hours in a year

There are 8760 hours in 1 year. To convert from hours to years, divide by 8760. To convert from years to hours, multiply by 8760.

How to calculate the passage of time

To calculate the passage of time, use the steps below. Given two times, Time A and Time B, where Time A occurs before Time B, use the steps below to calculate how much time has passed between Time A and Time B:

  1. If the time(s) are in 12-hour time notation, convert them to 24-hour time notation.

  2. Find the number of minutes from Time A to the next hour (or minute if calculating minutes and seconds).

  3. Find the number of hours from the time in step 1 to the last whole hour before Time B.

  4. Find the number of minutes from the whole hour in step 2 to Time B.

  5. Find the sum of steps 2-4. If the sum of the number minutes from steps 2 and 4 is greater than 60, subtract 60 from the number of minutes. The difference is the number of minutes, and we add 1 additional hour to the number of hours from step 3.


Find the number of hours and minutes between 4:11 PM and 9:17 PM.

  1. 4:11 is 16:11 and 9:17 is 21:17 in 24-hour time.

  2. The number of minutes from 16:11 to the next hour (17:00) is 60-11 = 49 minutes

  3. From 17:00 to 21:00, there are 4 hours.

  4. From 21:00 to 21:17, there are 17 minutes.

  5. The sum of 49 and 17 is 66, or 1 hour 6 minutes. The sum of 1 hour 6 minutes and 4 hours is 5 hours 6 minutes.

Thus, there are 5 hours and 6 minutes between 4:11 PM and 9:17 PM.

How to add hours and minutes

To add hours and minutes, use the following steps:

  1. Add all the minutes.

  2. If the sum of the minutes is greater than 60, convert to hours and minutes by subtracting 60 minutes (1 hour) from the sum as many times as necessary until the difference is less than 60. The final difference is the number of minutes. The number of hours is the number of times 60 was subtracted before the difference was less than 60. for example, given 121 minutes, we can subtract 60 twice and be left with 1 such that 121 minutes is 2 hours and 1 minute.

  3. Find the sum of all of the whole hours.

  4. Add the minutes.


The following are times in hours and minutes. Find their sum: 3:57, 4:12, 6:15.

  1. The sum of the minutes is 57 + 12 + 15 = 84

  2. 84 - 60 = 24. Thus, there are 1 hour 24 minutes.

  3. The sum of the whole hours (including the 1 from step 2) is 3 + 4 + 6 + 1 = 14

  4. Adding in the minutes, the sum is 14 hours 24 minutes.

Hours on a clock

The time of day in English is usually expressed in terms of hours. For example "10 o'clock" refers to both 10 AM and 10 PM. All whole hours can be expressed by adding "o'clock" after the number, though it can be unclear whether you mean the day or nighttime hour. This is why the system that uses the 24-hour clock, sometimes referred to as "military time," is the standard in most parts of the world and is an international standard.

In the 24-hour clock, the hours start at 0 for midnight and count up to 23, for 11 PM. When using this system, there are a number of different conventions for saying the time out loud depending on the context and location. In the US, which uses a combination of the 12-hour and 24-hour clock, the hours 12 and below are often still said as they would be in the 12-hour system. In some cases, people may still say "fifteen o'clock" for times above 12, but this is less common in the US.

Pronouncing military time

Since the 24-hour clock is often referred to as military time, it is common to pronounce times in the 24-hour system as the military does. In this method, the hour is always said first, including the zero if the hour is less than 10. The zero is pronounced as "oh." Whole hours are usually pronounced by saying the hour, followed by "hundred hours." For example, 08:00 would be pronounced "oh eight hundred hours," and 23:00 would be pronounced "twenty-three hundred hours." The "hours" is not mandatory though, and in some cases people may just say "oh eight hundred." Hundred is always used though, even for times such as 10:00 and 20:00, we would not say "one-thousand hours" or "two-thousand."

Although this may seem confusing or difficult to navigate, in most cases when speaking in the context of time, most people will understand the time you are trying to communicate, regardless how you say it.

Examples of how a person may pronounce time in the 24-hour system:

As you can see, there are many different variations on how to pronounce time on the 24-hour clock, but if you want to make it as clear as possible, within the context of talking about the time, you can say each individual number in the time.

Did you know?

Hours were not always counted starting at midnight (00:00). Back when artificial light didn't exist, a lot of societies based their time systems around the daylight and night hours. Many would count hours starting from sunrise with the middle of the day being the end of the sixth hour. Because of this, hours also used to vary in length because of changes in the number of daylight and night hours caused by changes in the season, as well as differences in location. These hours were called temporal, seasonal, or unequal hours. In some cases, people counted hours starting from sunset, or even midday.

It wasn't until mechanical clocks and the measurement of time became more exact that it became common to break days up based on their number of equal hours, rather than using easily identifiable points in the day (sunrise, noon, sunset) and unequal hours to segment the day.