Data compression ratio

Data compression ratio, also known as compression power, is a computer-science term used to quantify the reduction in data-representation size produced by a data compression algorithm. The data compression ratio is analogous to the physical compression ratio used to measure physical compression of substances, and is defined in the same way, as the ratio between the compressed size and the uncompressed size: [1]

 {\rm Compression\;Ratio} = \frac{\rm Compressed\;Size}{\rm Uncompressed\;Size}

Thus a representation that compresses a 10MB file to 2MB has a compression ratio of 2/10 = 0.2, often notated as an explicit ratio, 1:5 (read "one to five"), or as an implicit ratio, 1/5. Note that this formulation applies equally for compression, where the uncompressed size is that of the original; and for decompression, where the uncompressed size is that of the reproduction.

Sometimes the space savings is given instead, which is defined as the reduction in size relative to the uncompressed size:

{\rm Space\;Savings} = 1 - \frac{\rm Compressed\;Size}{\rm Uncompressed\;Size}

Thus a representation that compresses a 10MB file to 2MB would yield a space savings of 1 - 2/10 = 0.8, often notated as a percentage, 80%.

For signals of indefinite size, such as streaming audio and video, the compression ratio is defined in terms of uncompressed and compressed data rates instead of data sizes:

 {\rm Compression\;Ratio} = \frac{\rm Compressed\;Data\;Rate}{\rm Uncompressed\;Data\;Rate}

and instead of space savings, one speaks of data-rate savings, which is defined as the data-rate reduction relative to the uncompressed data rate:

{\rm Data\;Rate\;Savings} = 1 - \frac{\rm Compressed\;Data\;Rate}{\rm Uncompressed\;Data\;Rate}

For example, uncompressed songs in CD format have a data rate of 16 bits/channel x 2 channels x 44.1 kHz ≅ 1.4 Mbit/s, whereas AAC files on an iPod are typically compressed to 128 kbit/s, yielding a compression ratio of 0.09, for a data-rate savings of 0.91, or 91%.

When the uncompressed data rate is known, the compression ratio can be inferred from the compressed data rate.

Note: There is some confusion about the term 'compression ratio', particularly outside academia and commerce. In particular, some authors use the term 'compression ratio' to mean 'space savings', even though the latter is not a ratio; and others use the term 'compression ratio' to mean its inverse, even though that equates higher compression ratio with lower compression.

Lossless compression of digitized data such as video, digitized film, and audio preserves all the information, but can rarely do much better than 1:2 compression because of the intrinsic entropy of the data. In contrast, lossy compression (for example JPEG, or MP3) can achieve much higher compression ratios at the cost of a decrease in quality, as visual or audio compression artifacts from loss of important information are introduced.


  1. ^ [Data Compression: The Complete Reference. 4th Edition. David Salomon (with contributions by Giovanni Motta and David Bryant). Published by Springer (Dec 2006). ISBN 1-84628-602-6.]

External links

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Compression ratio — For compression ratio in data compression, see data compression ratio. The compression ratio of an internal combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its… …   Wikipedia

  • Data compression — Source coding redirects here. For the term in computer programming, see Source code. In computer science and information theory, data compression, source coding or bit rate reduction is the process of encoding information using fewer bits than… …   Wikipedia

  • compression ratio — The ratio of the original size of data that is sent to the compressed size. For example, a 3:1 compression ratio means that the original data takes up 3 times the amount of space as the compressed data, and a modem would transfer the data 3 times …   Dictionary of telecommunications

  • compression ratio — noun a) The ratio of the volume between the cylinder head of an internal combustion engine and the piston before and after the compression stroke b) The ratio of the size of compressed data after the execution of some compression algorithm to the …   Wiktionary

  • Compression Ratio — HD The difference between the original amount of data and the amount of data after the bandwidth has been reduced through compression, or the degree to which the data set has been reduced numerically. Usually expressed as a ratio such as 5:1 (5… …   Audio and video glossary

  • Lossless data compression — is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be… …   Wikipedia

  • Universal code (data compression) — In data compression, a universal code for integers is a prefix code that maps the positive integers onto binary codewords, with the additional property that whatever the true probability distribution on integers, as long as the distribution is… …   Wikipedia

  • Smart Data Compression — is a technique to compress GIS dataset, store all types of feature data and attribute information togetheras a core data structure, SDC format is used in ArcGIS streetmap, routeServer, routeMap IMS, business analyst, businessMap, ArcMobile SDK… …   Wikipedia

  • Smart Data Compression — Pour les articles homonymes, voir SDC. Smart Data Compression (dont l’extension est .sdc) est un format propriétaire de fichier utilisé pour le stockage d’information géographique. Un fichier SDC est destiné à être utilisé par des systèmes… …   Wikipédia en Français

  • Compression — may refer to: In physical science Compression (physical), the result of the subjection of a material to compressive stress Compression member, a class of structural elements, of which a column is the most common specific example Compressibility,… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”