# Content-Length header > Indicates the size of the request or response body in bytes. Source: https://apiguide.dev/headers/content-length Direction: both Registered: yes Defined in: RFC 9110, Section 8.6 Updated: 2026-10-14 ## What is the Content-Length header? The `Content-Length` header indicates the size of the payload body in decimal number of octets (bytes) sent to the recipient. ## API Usage & Best Practices * **Validate request sizing**: Gateways check this header to immediately reject payloads exceeding limits with a `413 Content Too Large`. * **Reject missing lengths only if you must**: RFC 9112 lets a server answer a request that has a body but no `Content-Length` with `411 Length Required`, but every HTTP/1.1 recipient must be able to parse chunked bodies, so reserve 411 for a server that genuinely cannot accept a body of unknown length, such as one that has to enforce a size limit before reading. An explicit `Content-Length: 0` is a valid, empty body rather than a missing header, and shouldn't trigger 411. ## Examples ```http POST /v1/leads HTTP/1.1 Content-Type: application/json Content-Length: 124 ```