Content-Length 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.

The HTTP Content-Length header indicates the size, in bytes, of the message body sent to the recipient.

Content-Length is limited in that the message size must be known up front, before sending the headers, which is a problem when content is dynamically generated or streamed.

  • In HTTP/1.0, it is required.
  • In HTTP/1.1, it could be replaced with Transfer-Encoding: chunked for responses sent out in parts as its size is calculated.
  • In HTTP/2, Content-Length is redundant, because the content length may be inferred from DATA frames. It may still be included for backwards compatibility.
Header type Request header, Response header, Content header
Forbidden request header Yes
CORS-safelisted response header Yes

Syntax

http
Content-Length: <length>

Directives

<length>

The length in octets.

Specifications

Specification
HTTP Semantics
# field.content-length

See also