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Common units of storage

Introduction
There are a number of units we need to know when talking about storage and memory size.

Bit
The basic building block of any computer is the switch. Computers, however, have millions and millions and millions of electronic switches in them, held in components such as RAM or the processor. Each switch can have one of two positions, on or off, which in computing, we represent as 1 or 0. Each switch can therefore hold one very simple piece of information (1 or 0) and we call each switch a 'bit' (from Binary digIT) and is the smallest unit of storage or memory that you can have. However, when you group these switches together in a certain way, you can represent data as binary codes, such as letters of the alphabet or numbers!

Nibbles and bytes
A single bit cannot hold a great range of numbers! It can hold either zero or one. You may have read about nibbles. A nibble is a group of 4 bits. The smallest value a nibble can hold is 0000 in binary and the largest number is 1111 in binary. (0000 in binary is the same as 0 in denary. 1111 in binary is the same as (1 x 8) + (1 x 4) + (1 x 2) + (1 x 1) or 15 in denary. It is also very common to group bits together in groups of 8. A group of eight bits is known as a 'byte'. Bytes are extremely convenient and important units of storage and memory to work with, as you will find out in due course.

Kilobytes, Megabytes, Gigabytes and Terabytes
We have seen that a byte can be used to represent a number. We will see soon that the number can be thought of as a code that represents a character on a keyboard. Before we look at that, however, we should note that if one byte is going to represent one character on the keyboard then we are going to have to collect together lots of bytes to record a memo, for example. For that reason, we frequently talk about Kilobytes, Megabytes and Gigabytes.

• • 1 Kilobyte (1 Kbyte) is 1024 bytes exactly, or 2¹⁰ bytes exactly, or about 1000 bytes, or about a thousand bytes.
  • 1 Megabyte (1 Mbyte) is 1048576 bytes exactly, or 2²⁰ bytes exactly, or about 1000 000 bytes, or about a million bytes.
  • 1 Gigabyte (1 Gbyte) is 1073741824 bytes exactly, 2³⁰ bytes exactly, or about 1000 000 000 bytes, or about a thousand million bytes.
  • 1 Terabyte (1 Tbyte) is 1099511627776 bytes exactly, 2⁴⁰ bytes exactly, or about 1000 000 000 000 bytes, or about a million million bytes.

So 15 Kbytes is about 15 thousand bytes. 128 Mbytes is about 128 million bytes. 20 Gbytes is about 20 thousand million bytes and 5 Tbytes is about 5 billion bytes (or 5 million million bytes, if you prefer). More often than not, you don't need to know the exact number of bytes, just an approximation!

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