HTTP Status Codes: Complete Reference Guide

Understand every HTTP response status code. From 100 Continue to 511 Network Authentication Required, this comprehensive reference covers what each code means, when to use it, and common pitfalls.

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Why HTTP Status Codes Matter

Every HTTP response from a web server includes a three-digit status code that communicates the outcome of the request. These codes are the universal language of the web, used by browsers, APIs, CDNs, load balancers, and monitoring tools to understand what happened during a request. For web developers, API designers, and DevOps engineers, a thorough understanding of HTTP status codes is essential for building reliable applications, debugging failed requests, designing intuitive APIs, and maintaining healthy web services. Using the wrong status code can confuse clients, break caching, harm SEO rankings, and make debugging significantly harder. This guide provides a complete reference for every standard HTTP status code along with practical guidance on when and how to use each one.

The Five Classes of HTTP Status Codes

HTTP status codes are organized into five classes based on their first digit. Each class represents a broad category of response type.

ClassRangeMeaningCommon Examples
1xx Informational100-199Request received, continuing process100 Continue, 101 Switching Protocols
2xx Success200-299Request successfully received and processed200 OK, 201 Created, 204 No Content
3xx Redirection300-399Further action needed to complete request301 Moved Permanently, 304 Not Modified
4xx Client Error400-499Request contains bad syntax or cannot be fulfilled400 Bad Request, 404 Not Found, 429 Too Many Requests
5xx Server Error500-599Server failed to fulfill a valid request500 Internal Server Error, 503 Service Unavailable

Step-by-Step: Using the HTTP Status Code Lookup

1

Enter a Status Code

Open the HTTP Status Code Lookup and type any three-digit HTTP status code into the search field. You can enter codes like 200, 404, or 500 to instantly retrieve the full details.

2

Read the Description

The tool displays the official name of the status code, its class category, and a clear description of what the code means in the context of HTTP communication.

3

View Usage Guidelines

See practical guidance on when and how to use the status code, common scenarios where it appears, and best practices for handling it in your applications.

4

Explore Related Codes

Browse related status codes and compare similar codes to choose the right one for your use case. Switch between codes quickly without re-entering your search.

Essential Status Codes Every Developer Must Know

Most Common Success Codes: 200 OK - Standard successful response 201 Created - Resource was created successfully 204 No Content - Success but no response body 206 Partial Content - Partial resource returned (range requests) Most Common Redirect Codes: 301 Moved Permanently - Resource permanently moved 302 Found - Temporary redirect 304 Not Modified - Use cached version 307 Temporary Redirect - Preserve request method 308 Permanent Redirect - Preserve request method permanently Most Common Client Error Codes: 400 Bad Request - Malformed syntax 401 Unauthorized - Authentication required 403 Forbidden - Insufficient permissions 404 Not Found - Resource does not exist 405 Method Not Allowed - HTTP method not supported 409 Conflict - Resource state conflict 422 Unprocessable Entity - Validation errors 429 Too Many Requests - Rate limit exceeded Most Common Server Error Codes: 500 Internal Server Error - Unexpected server failure 502 Bad Gateway - Invalid upstream response 503 Service Unavailable - Server temporarily overloaded 504 Gateway Timeout - Upstream server too slow

Common Use Cases

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API Design

Choose the right status codes for your REST and GraphQL APIs to communicate response outcomes clearly to client applications and third-party integrators.

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Debugging

Quickly identify what went wrong during a failed HTTP request by looking up the status code returned by the server and understanding its meaning.

📈

SEO & Redirects

Use the correct redirect status codes (301, 302, 308) to manage site migrations, URL changes, and link equity preservation for search engine optimization.

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Error Handling

Build robust error handling in your applications by properly interpreting and responding to different status code categories in client and server code.

Best Practices for HTTP Status Codes

  • Use the most specific code available: Do not return 200 for errors or 500 for client mistakes. Use the most specific status code that accurately describes the response outcome.
  • Include a descriptive reason phrase: While the status code number is what matters programmatically, the reason phrase helps developers debugging with browser DevTools and curl.
  • Return 201 with a Location header: When creating resources via POST, return 201 with a Location header pointing to the newly created resource.
  • Use 422 for validation errors: When request syntax is valid but the data fails validation rules, return 422 Unprocessable Entity instead of 400 Bad Request.
  • Implement Retry-After for 429 and 503: When rate limiting or temporarily overloaded, include a Retry-After header to tell clients when they can retry.
  • Log and monitor status codes: Track the distribution of status codes in your applications. A spike in 5xx codes indicates server problems; a spike in 4xx codes may indicate client bugs or abuse.

Common Mistakes to Avoid

  • Returning 200 for errors: Some APIs return 200 with an error message in the body. This breaks standard HTTP tooling, monitoring, and makes debugging harder. Use appropriate 4xx or 5xx codes instead.
  • Using 302 for permanent redirects: 302 is temporary. Use 301 for permanent URL changes to preserve search engine rankings and allow clients to update their bookmarks.
  • Sending 401 without WWW-Authenticate: The 401 status code requires a WWW-Authenticate header indicating how to authenticate. Without it, clients do not know how to proceed.
  • Using 403 instead of 401: 401 means unauthenticated (not logged in); 403 means unauthorized (logged in but no permission). Mixing these up confuses clients about whether they need to log in or need more permissions.
  • Not handling 304 caching correctly: A 304 response means the cached version is still valid. Failing to implement proper cache headers alongside 304 responses can cause stale content issues.
  • Using 500 for all server errors: Be more specific when possible. Use 502 for bad gateway responses, 503 for temporary overload, 504 for upstream timeouts, and 500 only for truly unexpected failures.

Privacy and Security Considerations

HTTP status codes can reveal information about your server infrastructure and application behavior. Consider these security implications:

  • Avoid detailed error messages in 5xx responses: Stack traces and internal error details in server error responses can expose sensitive information about your infrastructure. Log these details server-side and return generic messages to clients.
  • Do not leak information through status codes: Returning different status codes for non-existent resources depending on whether they once existed (404 vs 410) can allow attackers to enumerate your data.
  • Implement rate limiting with 429: Protect your APIs from abuse by returning 429 Too Many Requests with appropriate Retry-After headers when clients exceed request limits.
  • Use 407 Proxy Authentication Required carefully: This code can reveal the presence of proxy servers in your architecture. Use it only when necessary.
  • Client-side lookup only: Our HTTP Status Code Lookup tool processes your query entirely in the browser. No data is sent to any server, ensuring complete privacy for your debugging sessions.

Frequently Asked Questions

What are the five classes of HTTP status codes?

HTTP status codes are divided into five classes: 1xx (Informational) indicates the request was received and is being processed; 2xx (Success) indicates the request was successfully received and processed; 3xx (Redirection) indicates further action is needed to complete the request; 4xx (Client Error) indicates the request contains bad syntax or cannot be fulfilled; 5xx (Server Error) indicates the server failed to fulfill a valid request.

What is the difference between 301 and 302 redirects?

301 Moved Permanently indicates the resource has permanently moved to a new URL. Search engines transfer link equity to the new URL. 302 Found is a temporary redirect; the original URL should be used again. Search engines keep the original URL indexed. Use 301 for permanent changes and 302 for temporary ones.

What does HTTP 429 Too Many Requests mean?

HTTP 429 indicates the client has sent too many requests in a given time period (rate limiting). The server may include a Retry-After header specifying how long to wait before retrying. This is commonly used in APIs to prevent abuse and ensure fair resource usage across clients.

What is the difference between 401 and 403?

401 Unauthorized means the client is not authenticated (no valid credentials). The server should respond with a WWW-Authenticate header indicating how to authenticate. 403 Forbidden means the client is authenticated but does not have permission to access the resource. Authentication succeeds but authorization fails.

Should I use 404 or 410 for deleted content?

Use 404 Not Found when the resource does not exist or its existence is unknown. Use 410 Gone when the resource has been permanently deleted and will never be available at that URL again. 410 is more specific and tells search engines to stop indexing the URL.

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