# Caribbean Secondary Education Certificate - Information Technology/Units of Storage

## Units of Storage

What are bits, bytes, and other units of measure for digital information?

A bit is a binary digit, the smallest increment of data on a computer. A bit can hold only one of two values: 0 or 1, corresponding to the electrical values of off or on, respectively.

Because bits are so small, you rarely work with information one bit at a time. Bits are usually assembled into a group of eight to form a byte. A byte contains enough information to store a single ASCII character, like "h".

A kilobyte (KB) is 1,024 bytes, not one thousand bytes as might be expected, because computers use binary (base two) math, instead of a decimal (base ten) system.

Computer storage and memory is often measured in megabytes (MB) and gigabytes (GB). A medium-sized novel contains about 1MB of information. 1MB is 1,024 kilobytes, or 1,048,576 (1024x1024) bytes, not one million bytes.

Similarly, one 1GB is 1,024MB, or 1,073,741,824 (1024x1024x1024) bytes. A terabyte (TB) is 1,024GB; 1TB is about the same amount of information as all of the books in a large library, or roughly 1,610 CDs worth of data. A petabyte (PB) is 1,024TB. Indiana University is now building storage systems capable of holding petabytes of data. An exabyte (EB) is 1,024PB. A zettabyte (ZB) is 1,024EB. Finally, a yottabyte (YB) is 1,024ZB.

Many hard drive manufacturers use a decimal number system to define amounts of storage space. As a result, 1MB is defined as one million bytes, 1GB is defined as one billion bytes, and so on. Since your computer uses a binary system as mentioned above, you may notice a discrepancy between your hard drive's published capacity and the capacity acknowledged by your computer. For example, a hard drive that is said to contain 10GB of storage space using a decimal system is actually capable of storing 10,000,000,000 bytes. However, in a binary system, 10GB is 10,737,418,240 bytes. As a result, instead of acknowledging 10GB, your computer will acknowledge 9.31GB. This is not a malfunction but a matter of different definitions.

We count in base 10 by powers of 10:

 101 = 10
102 = 10*10 =  100
103 = 10*10*10 = 1,000
106 = 1,000,000


Computers count by base 2:

 21 = 2
22 = 2*2 = 4
23 = 2*2*2 = 8
210 = 1,024
220 = 1,048,576


So in computer jargon, the following units are used:

Unit Equivalent 1 kilobyte (KB) 1,024 bytes 1 megabyte (MB) 1,048,576 bytes 1 gigabyte (GB) 1,073,741,824 bytes 1 terabyte (TB) 1,099,511,627,776 bytes 1 petabyte (PB) 1,125,899,906,842,624 bytes

Note: The names and abbreviations for numbers of bytes are easily confused with the notations for bits. The abbreviations for numbers of bits use a lower-case "b" instead of an upper-case "B". Since one byte is made up of eight bits, this difference can be significant. For example, if a broadband Internet connection is advertised with a download speed of 3.0Mbps, its speed is 3.0 megabits per second, or 0.375 megabytes per second (which would be abbreviated as 0.375MBps). Bits and bit rates (bits over time, as in bits per second [bps]) are most commonly used to describe connection speeds, so pay particular attention when comparing Internet connection providers and services.