Compression ratio

Compression ratio compares the size of data after compression with its size before compression. It is commonly expressed as original size divided by compressed size, though conventions vary.

Lossless compression: A method that reduces data size while allowing exact reconstruction of the original. Its ratio measures size reduction without sacrificing any original information.
File compression: The reduction of a file's storage size through an encoding that may be reversible or lossy. File utilities report ratios to summarize how much a particular file shrank.
ZIP (file format): An archive format that stores files using lossless compression and metadata. ZIP archives make the relationship between source files and compressed size directly measurable.
Compression efficiency: A broad assessment of how effectively a method reduces data under specified constraints. Efficiency can account for more than the size ratio alone.
Lossy compression: A method that reduces data size by discarding information that may be imperceptible or unimportant. Its ratios can be higher because reconstruction need not preserve every original detail.
Image compression: The reduction of image data using reversible or perceptually lossy encoding methods. Image ratios vary with image content, format, and acceptable visual distortion.
GZIP: A file-compression format and utility built around the DEFLATE algorithm. GZIP reports size changes for individual data streams and commonly compressible files.
Compression speed: The rate at which an algorithm compresses or decompresses data. Two algorithms with similar ratios may require very different processing time.
Data redundancy: Repeated or predictable information that can be represented more compactly. Redundancy gives compression algorithms material to remove or encode efficiently.
Audio compression: The reduction of digital audio data, often by exploiting limits of human hearing. Audio formats trade file size against fidelity in ways summarized by compression ratios.

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