HTTP Status Code Reference — Complete Guide to All HTTP Response Codes

This complete HTTP status code reference covers every HTTP response code from 100 to 511, including their meaning, when they are used, and common causes. Bookmark this page for quick reference when debugging web applications, APIs, load balancers, reverse proxies and CDNs.

HTTP Status Code Categories

• 1xx — Informational: The request was received and is being processed. Rarely seen by end users.
• 2xx — Success: The request was received, understood and accepted. 200 OK is the most common.
• 3xx — Redirection: Further action is needed to complete the request. Used for URL redirects and caching.
• 4xx — Client Error: The request contains bad syntax or cannot be fulfilled. The client is at fault.
• 5xx — Server Error: The server failed to fulfil a valid request. The server is at fault.

The Most Important HTTP Status Codes

• 200 OK — Standard success response. Everything worked.
• 201 Created — Resource successfully created (returned after POST requests in REST APIs)
• 301 Moved Permanently — URL has changed permanently. Browsers and search engines update their links. Use for SEO-safe redirects.
• 302 Found — Temporary redirect. Search engines keep the original URL.
• 400 Bad Request — Malformed request syntax, invalid parameters
• 401 Unauthorized — Authentication required (not yet authenticated)
• 403 Forbidden — Authenticated but lacks permission
• 404 Not Found — Resource doesn't exist at this URL
• 429 Too Many Requests — Rate limit exceeded
• 500 Internal Server Error — Generic server-side failure
• 502 Bad Gateway — Upstream server returned invalid response (common with nginx/load balancers)
• 503 Service Unavailable — Server temporarily unable to handle requests (maintenance, overload)
• 504 Gateway Timeout — Upstream server didn't respond in time

HTTP Status Codes and SEO

Status codes directly affect SEO. 301 redirects pass approximately 90–99% of link equity to the new URL — use them for permanent moves. 302 redirects are temporary — search engines keep indexing the original URL. 404 errors on pages with backlinks waste link equity — redirect them to relevant content. 503 with Retry-After tells Googlebot to come back later during planned maintenance.