Content-Encoding header
Baseline Widely available *
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
* Some parts of this feature may have varying levels of support.
The HTTP Content-Encoding representation header lists the encodings and the order in which they have been applied to a resource.
This lets the recipient know how to decode the data in order to obtain the original content format described in the Content-Type header.
Content encoding is mainly used to compress content without losing information about the original media type.
Servers should compress data as much as possible, and should use content encoding where appropriate.
Compressing already compressed media types, such as .zip or .jpeg, is usually not appropriate because it can increase the file size.
If the original media is already encoded (e.g., as a .zip file), this information is not included in the Content-Encoding header.
When the Content-Encoding header is present, other metadata (e.g., Content-Length) refer to the encoded form of the data, not the original resource, unless explicitly stated.
Content encoding differs to Transfer-Encoding in that Transfer-Encoding handles how HTTP messages themselves are delivered across the network on a hop-by-hop basis.
| Header type | Representation header |
|---|---|
| Forbidden request header | No |
Syntax
Content-Encoding: gzip
Content-Encoding: compress
Content-Encoding: deflate
Content-Encoding: br
Content-Encoding: zstd
Content-Encoding: dcb
Content-Encoding: dcz
// Multiple, in the order in which they were applied
Content-Encoding: deflate, gzip
Directives
gzip-
A format using the Lempel-Ziv coding (LZ77), with a 32-bit CRC. This is the original format of the UNIX gzip program. The HTTP/1.1 standard also recommends that the servers supporting this content-encoding should recognize
x-gzipas an alias, for compatibility purposes. compress-
A format using the Lempel-Ziv-Welch (LZW) algorithm. The value name was taken from the UNIX compress program, which implemented this algorithm. Like the compress program, which has disappeared from most UNIX distributions, this content-encoding is not used by many browsers today, partly because of a patent issue (it expired in 2003).
deflate-
Using the zlib structure (defined in RFC 1950) with the deflate compression algorithm (defined in RFC 1951).
br-
A format using the Brotli algorithm structure (defined in RFC 7932).
zstd-
A format using the Zstandard algorithm structure (defined in RFC 8878).
dcbExperimental-
A format that uses the Dictionary-Compressed Brotli algorithm. See Compression Dictionary Transport.
dczExperimental-
A format that uses the Dictionary-Compressed Zstandard algorithm. See Compression Dictionary Transport.
Examples
Compressing with gzip
On the client side, you can advertise a list of compression schemes that will be sent along in an HTTP request. The Accept-Encoding header is used for negotiating content encoding.
Accept-Encoding: gzip, deflate
The server responds with the scheme used, indicated by the Content-Encoding response header.
Content-Encoding: gzip
Whether a server uses compression methods requested by the client depends on server configuration and capabilities.
Specifications
| Specification |
|---|
| HTTP Semantics # field.content-encoding |
| Zstandard Compression and the 'application/zstd' Media Type # name-content-encoding |