Units of Data Storage (OCR GCSE Computer Science)

Revision Note

Written by: Robert Hampton

Reviewed by: James Woodhouse

Computers use binary numbers to represent data
Data such as characters, images and sound must be stored as binary
The smallest unit of data a computer can store is 1 binary digit, otherwise expressed as 1 bit
1 bit can only hold one value (2¹), this is not big enough to store all kinds of data, so computers have different 'Units of Data'

A unit of data is a term given to describe different amounts of binary digits stored on a digital device
These are the units you need to know for GCSE:

Unit	Symbol	Binary	Written as	Example
Bit	b	1 or 0
Nibble		4 b
Byte	B	8 b		A single character
Kilobyte	KB	1000 B (2¹⁰)	Thousand bytes	A small text file
Megabyte	MB	1000 KB (2²⁰)	Million byes	A music file
Gigabyte	GB	1000 MB (2³⁰)	Billion bytes	A high definition movie
Terabyte	TB	1000 GB (2⁴⁰)	Trillion bytes	A large hard drive
Petabyte	PB	1000 TB (2⁵⁰)	Quadrillion bytes	A large data centre

Binary only contains two digits (1 and 0) so technically larger multiples would be calculated as 2^{number of bytes}

For example, a kilobyte is 2¹⁰= 1024 bytes not 1000 bytes

In GCSE we approximate all larger units of storage as multiples of 1000 to make calculations easier

It is often a requirement of the exam to be able to convert between different units of data, for example bytes to megabytes (larger) or kilobytes to bytes (smaller)
This process involves division, moving up in size of unit and multiplication, moving down in size of unit
When dealing with all units bigger than a byte we use multiples of 1000
For example, 2000 kilobytes in megabytes would be 2000 / 1000 = 2 MB and 2 terabytes in gigabytes would be 2 * 1000 = 2000 GB
When dealing with bits and bytes the same process is used with the value 8 as there are 8 bits in a byte
For example, 24 bits in bytes would be 24 / 8 = 3 B and 10 bytes in bits would be 10 * 8 = 80 b