Bits and Bytes

A BIT is one number in binary code--a 0 or a 1. "Bit" stands for "BInary digiT."

For example, "0" is a bit. "1" is also a bit. "10" is two bits. "101" is three bits, and so on.

A BYTE is eight bits. For example, 11110000 is a byte. 10101010 is also a byte. 11010010 is also a byte. Any combination of eight ones and zeroes is a byte. In a byte, you have 256 possible numbers. The first number is 0 ("00000000"), and the highest number is 255 ("11111111").

Why do we use the byte? Why not only count in bits?

Well, the byte can be useful. For example, people use letters and numbers and symbols and punctuation. These are called "characters." How can a computer understand that? It only knows 1s and 0s. Well, we use a special code called "ASCII" to translate between binary numbers and characters. Each character has its own 8-digit binary number ("byte"). For example, the lowercase letter "a" has the binary code "01100001." Here is a chart with some sample ASCII binary codes (see this page for a larger chart):

 Character ASCII Binary Code
A 0100 0001
B 0100 0010
C 0100 0011
D 0100 0100
a 0110 0001
b 0110 0010
c 0110 0011
d 0110 0100

Can you see how that works? Every letter, number, symbol and punctuation mark has a 1-byte binary code. This 1-byte system was convenient, so it was used in many other places, and became common.


So where do you use bits and where do you use bytes?

Usually, bytes are used to describe the amount of stored data--that is, how much data on a floppy disk, CD-ROM or hard drive.

Usually, bits are used to describe transmission speeds--how many bits of data are sent in one second.

You can tell bits and bytes apart by how they are written: a bit is shown as a small, lower-case "b". A byte uses an upper-case capital "B". Therefore, a Mb is a megabit, and a MB is a megabyte. Remember, a byte equals 8 bits. So, 1 MB = 8Mb.


If you want to know how big a computer file is, you have to use BYTES. Here is a chart of the different types of counted bytes.

Term  Abbreviation Number of Bytes*
Byte  B 1
Kilobyte KB (K) 1,000
Megabyte MB 1,000,000
Gigabyte GB 1,000,000,000
Terabyte TB 1,000,000,000,000
The numbers are not exact; they are all powers of 2.
A kilobyte is not really 1000 bytes, it is 1024. We say "1000" in base 10 for convenience.

Notice that the numbers use the binary prefixes--kilo, mega, giga, tera, peta, exa, zetta, yotta, etc. They are explained simply here.

So what do these sizes mean? Well, a simple text file made by a word processing program could be just 20K in size. A larger text file could be 200K in size. A photograph taken by a digital camera might be 1MB. An MP3 music file might be 5 MB. An MPG movie file might be 1GB in size. These are just examples; the size of each file could be much smaller or much larger depending on how much content they hold.

Here is an important word to remember: Capacity. "Capacity" means the maximum amount that something can hold; for example, the capacity of an elevator might be 12 persons, or the capacity of a bottle of water might be 500 ml. In computing "capacity" means how many Bytes can be held in a storage area.

You might also compare these figures with the capacities of common storage media:

Type Capacity
Floppy 1.4 MB
CD 700 MB
DVD 4.7 GB
Blu-Ray 25, 50, 100 GB
HDD up to 2 TB (and growing)


When you want to describe how fast data is sent between computers, you use bits per second (bps).

Term  Abbreviation Number Real Number of Bits
bit  bps 1 1
Kilobit Kbps 1,000 1,024
Megabit Mbps 1,000,000 1,048,576
Gigabit Gbps 1,000,000,000 1,073,741,824

Also remember that 8 Mbps means you can send 8 MegaBITs, which equals 1 MegaBYTE.

You have probably heard "bits" before, when you heard about the speed of Internet connections. When you get a DSL connection at home, the tell you the speed in "Megabits," for example, 50Mbps (50 Megabits per second).

It is important to remember: 8 bits is 1 Byte!