A data compression format is a defined way of representing compressed data so that compatible software can decode it. The format fixes how compressed bytes are structured and interpreted. It may preserve the original data exactly (lossless) or reconstruct only an approximation of it (lossy). Some formats are general-purpose, and others are built for a particular medium such as audio or images.
What a compression format actually specifies
A compression format is a contract between the software that writes compressed data and the software that reads it. The specification says how the compressed stream is laid out, what each part means, and how a decoder must turn it back into usable data. Two programs that follow the same specification can exchange data without knowing anything about each other’s internals.
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Many specifications stop at the compressed stream itself. Others also define the wrapper around it. RFC 8878, which describes the Zstandard format, is an example of the second kind: alongside the format it registers a media type and a content encoding, so that transport systems can identify the data. The zlib specification in RFC 1950 defines an interoperable lossless compressed data format. Specifications therefore differ in how much surrounding structure they cover, and a reader should check which layer a given document describes.
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Lossless and lossy: the distinction that matters most
The clearest standards-based definition of lossless compression appears in RFC 9639, the specification for the Free Lossless Audio Codec (FLAC). It describes lossless compression as “reducing the amount of computer storage space needed to store data without needing to remove or irreversibly alter any of this data in doing so.”
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The same specification explains the opposite case. Lossy compression removes, irreversibly alters, or approximates information, so decoding returns an approximation of the original rather than the original itself. The practical consequence is that a lossless format can reproduce the source byte for byte, while a lossy format trades fidelity for a smaller result. Whether that trade is acceptable depends entirely on the data. A decompressed text file or program must match its source exactly; a photograph or a piece of music may tolerate small, deliberate differences.
Lossy formats are not defined by a single algorithm. Each format specifies its own loss behavior, and some formats support both modes, as the examples below show.
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Format, container and method: three terms that overlap
Readers often encounter these three words used loosely, and the distinction is worth stating precisely.
- Method (algorithm): the transformation that turns input into a smaller representation and back. Deflate-style and Zstandard-style methods are examples.
- Compressed stream format: the specified byte layout that carries the output of a method, including any header, checksum or block structure that the specification requires.
- Container or file format: framing and metadata wrapped around one or more streams, which may also hold non-compressed information. JPEG 2000 specifies a file format in addition to its codestream syntax, which is one case where a single standard defines both layers.
Because specifications use these terms differently, a file extension alone does not tell you which layer you are dealing with. Read the specification named by the software or standard, not the extension.
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Examples by kind of data
The table below compares formats from the specifications cited here. Capabilities are listed only where the specification states them; where a specification is silent on a point, the cell says so.
| Format | Specification | Target data | Loss behavior | Notes stated in the specification |
|---|---|---|---|---|
| zlib | RFC 1950 | General-purpose data | Lossless | Interoperable compressed data format; can use different compression methods; does not attempt random access |
| Brotli | RFC 7932 | General-purpose data | Lossless | Lossless compressed data format; does not attempt random access |
| Zstandard | RFC 8878 (February 2021; obsoletes RFC 8478) | General-purpose file compression and streaming | Lossless | Registers a media type and a content encoding |
| FLAC | RFC 9639 | Digital audio | Lossless | Reduces storage needed for digital audio without removing or irreversibly altering data |
| WebP | RFC 9649 | Still images | Lossy and lossless | Also supports transparency and animation |
| JPEG 2000 | ITU-T T.800, Version 4 (dated July 2024) | Digital still images | Lossless and lossy methods | Defines decoding processes, codestream syntax and a file format |
Two cautions follow from the table. First, a format that offers lossless compression for one kind of data does not imply that it offers lossy compression for another, and the reverse is also true. Second, the table is not a ranking. The specifications describe design properties and constraints; they do not establish which format compresses best or runs fastest, and no current comparable benchmark from these sources supports such a claim.
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How to compare formats
When more than one format is a candidate, the following questions separate them more reliably than a general reputation does.
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- Data type: is the format general-purpose, or specialized for images, audio or another medium?
- Processing model: does the specification support streaming or sequential input, and does it provide random access? The zlib and Brotli specifications both state that they do not attempt random access, which matters if you need to read one part of a large stream without decoding the rest.
- Interchange and identification: does the specification define framing, file structure or transport identifiers? RFC 8878 includes media type and content encoding registrations, which affects how web and transport systems label the data.
- Compatibility: can the software that receives the data decode that exact format and the options selected when it was written? This is a practical requirement for your own systems. It is not a measured ranking of how widely each format is supported.
Practical checks before choosing a format
- Confirm whether the data must be restored exactly. If it must, use only formats whose specification defines lossless operation for that data type.
- Identify the specification that governs the format you intend to use, and note its number and date.
- Check whether the receiving software implements that specification, including any optional modes you plan to use.
- If you need random access or streaming, confirm the specification’s processing model before assuming it applies.
Specifications are revised and superseded over time. The Zstandard specification in RFC 8878, for example, replaced RFC 8478, and JPEG 2000 has a dated edition in ITU-T T.800. When you rely on a particular revision, cite it explicitly and confirm that the software you use follows it.
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