Storage Units, Capacity and Compression

Units of Digital Storage

Every piece of data in a computer is ultimately made of bits. Bits are grouped into larger units so that storage capacity can be expressed at a human-readable scale. Your exam uses base-2 (binary) prefixes, which are defined in powers of 2, not powers of 10. This distinction matters: answers using base-10 values will lose marks in storage calculation questions.

Storage units — base-2 (binary) prefix definitions

Unit	Abbreviation	Exact value
Bit	b	1 binary digit (0 or 1)
Nibble	—	4 bits = 1 hex digit
Byte	B	8 bits
Kilobyte	KB	1024 bytes = \(2^{10}\) bytes
Megabyte	MB	1024 KB = \(2^{20}\) bytes
Gigabyte	GB	1024 MB = \(2^{30}\) bytes
Terabyte	TB	1024 GB = \(2^{40}\) bytes
Petabyte	PB	1024 TB = \(2^{50}\) bytes

In everyday consumer products, manufacturers often count in powers of 10 instead: 1 kilobyte = 1000 bytes, 1 megabyte = 1,000,000 bytes, and so on — this is why a “500 GB” hard drive shows about 465 GB of space once it is formatted. Your exam uses the base-2 definitions: 1 KB = 1024 bytes, 1 MB = 1024 KB etc. Always use these in calculations.

Calculating Storage Capacity

Storage capacity questions often involve converting between units or working out how many files fit in a given space. The general method is to express both the capacity and the file size in the same unit, then divide. Always work in base-2 (multiply or divide by 1024 at each prefix step).

What Is Compression?

Compression reduces file size by encoding data more efficiently. There are two fundamentally different types: lossy and lossless. The right choice depends on whether the exact original data must be recovered.

Lossy Compression

Lossy compression permanently discards some data to achieve a much smaller file. The algorithm exploits limits of human perception: it removes detail that the eye or ear is unlikely to notice. Common lossy formats are JPEG (images) and MP3 (audio).

A JPEG might store a photograph at one-tenth the size of an uncompressed bitmap by discarding subtle colour variations in smooth gradients. An MP3 removes audio frequencies below the threshold of human hearing and reduces detail in frequencies masked by louder sounds. The reduction in quality is “lossy” because you can never recover the removed data — each time a lossy file is re-compressed, more data is lost.

Lossless Compression

Lossless compression reduces file size without losing any information. When the file is decompressed, the output is bit-for-bit identical to the original. Common lossless formats include PNG (images) and FLAC (audio), as well as ZIP for general data.

A simple lossless technique is run-length encoding (RLE): instead of storing “AAAAAABBB” as 9 separate characters, RLE stores “6A3B” (6 copies of A, then 3 copies of B). The compressed version is shorter, and the original can be reconstructed exactly. Lossless compression works best when there is a lot of repeated or predictable data.

Compression Ratio

The compression ratio expresses how much smaller a compressed file is compared to the original:

Compression ratio = original size ÷ compressed size

A ratio of 4:1 means the compressed file is one-quarter the size of the original. For example, if a 20 MB WAV audio file is compressed to a 2 MB MP3, the compression ratio is 20 ÷ 2 = 10:1. A higher ratio means greater size reduction, which usually also means greater quality loss for lossy formats.