Content-Length header

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 {{httpheader("Transfer-Encoding", "Transfer-Encoding: chunked")}}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	{{Glossary("[Request header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Request%20header), {{Glossary("[Response header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Response%20header), {{Glossary("[Content header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Content%20header)
{{Glossary("[Forbidden request header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Forbidden%20request%20header)	Yes
{{Glossary("[CORS-safelisted response header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted%20response%20header)	Yes

Syntax

Content-Length: <length>

Directives

• <length>
  • : The length in octets.

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• {{HTTPHeader("Transfer-Encoding")}}Encoding