Storage Units

For GCSE Computer Science you need to be aware of the units used for computer storage - bits, bytes, kilobytes, megabytes, gigabytes, terabytes and petabytes - and be able to convert between them.

Units

The units that you need to know are as follows:

• a bit is a single binary digit - 0 or 1
• a byte is eight bits
• a nibble† (sometimes nybble) is four bits (i.e. half a byte)
• a kilobyte is 1000 bytes
• a megabyte is 1000 kilobytes (or 1,000,000 / a million bytes)
• a gigabyte is 1000 megabytes (or 1,000,000,000 / a billion bytes)
• a terabyte is 1000 gigabytes (or 1,000,000,000,000 / a trillion bytes)
• a petabyte is 1000 terabytes (or 1,000,000,000,000,000 / a quadrillion bytes)

† I used to work in the software industry, and I've never heard anyone outside the classroom say nibble to mean four bits!

Apart from when you change from bits to bytes, a change in the prefix indicates that the size has changed by a factor of 1000 (or that the power has changed by three if you are using standard form).

Abbreviations

The prefixes used are the standard ones for the metric system, but there are no fractions of a byte, so no milli- or centibytes. Just as with chemical symbols, capitalisation is important:

• the kilo-, mega-, giga-, tera- and peta- prefixes are represented by the letters k, M, G, T, and P respectively - i.e. they are all upper case apart from k
• we use b for bits and B for bytes, e.g. kB is kilobytes and kb is kilobits

Note that we normally use bytes to measure file sizes, but bits to measure bandwidth (i.e. transfer speeds) - e.g. if your broadband is advertised as 40Mbps that's megabits per second (i.e. 5MBps).

Questions

Type your answers into the boxes and click the Check button to see whether they are correct. You will not be able to use a calculator in the GCSE exam, but you can here because the numbers might be bigger.