Units of data

A computer is built from billions of tiny electronic switches (transistors). Each switch is either on or off, which we write as 1 or 0. So everything a computer processes or stores, whether numbers, text, images, sound or the instructions of a program, must first be converted into binary: patterns of 1s and 0s.

The smallest unit of data is a single 1 or 0: a bit (short for binary digit).

To convert to a bigger unit, divide (by 1,000, or by 8 from bits to bytes). To convert to a smaller unit, multiply.

Example 1: how many megabytes are in 3 GB?

1. GB to MB is a smaller unit, so multiply: 3 × 1,000
2. = 3,000 MB (or 3 × 1,024 = 3,072 MB)

Example 2: a file is 24,000 bits. How big is it in kilobytes?

1. Bits to bytes: 24,000 ÷ 8 = 3,000 bytes
2. Bytes to KB: 3,000 ÷ 1,000 = 3 KB

Example 3: how many bits are in 2 KB, using 1,024?

1. 2 × 1,024 = 2,048 bytes
2. 2,048 × 8 = 16,384 bits

Storage devices have a fixed capacity, such as a 32 GB memory stick or a 2 TB hard drive. To check that some files will fit, convert them all to the same unit, then add them up.

Example: will 400 photos of 4 MB each and 50 songs of 6 MB each fit on a 2 GB memory stick?

1. Photos: 400 × 4 = 1,600 MB
2. Songs: 50 × 6 = 300 MB
3. Total: 1,600 + 300 = 1,900 MB = 1.9 GB
4. 1.9 GB is less than 2 GB, so yes, just.

Real files are a little bigger, as they also store metadata (information about the file), and the exam may let you allow for this.

To work out the size of a sound, image or text file, see those sub-topics.