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HTTP status codes are three-digit responses grouped by their first digit: 1xx means informational, 2xx success, 3xx redirection, 4xx client error, and 5xx server error. That class is only a starting point—the individual code gives the more specific registered meaning, and the applicable HTTP specification determines how clients and servers should handle it.
What the five HTTP status-code classes mean
The IANA HTTP Status Code Registry describes the five classes as follows. These are broad categories, not diagnoses of what happened inside a particular API or website.
| Class | Registry summary | How to read it |
|---|---|---|
| 1xx | Informational: request received; processing is continuing. | An interim response, not usually the final result of the request. |
| 2xx | Success: the action was successfully received, understood, and accepted. | The request succeeded at the HTTP level; check the specific code and response content for what the application did. |
| 3xx | Redirection: further action must be taken to complete the request. | The client may need to make another request, following the specific redirect semantics. |
| 4xx | Client error: the request contains bad syntax or cannot be fulfilled. | Investigate the request, credentials, permissions, resource, or request conditions; the exact code narrows the issue. |
| 5xx | Server error: the server failed to fulfill an apparently valid request. | Investigate the responding server or an upstream dependency; a 5xx alone does not identify the failing component. |
Common HTTP status codes: quick reference
This is a practical selection of registered codes, not every assigned value. Names and registration status below follow the IANA registry, whose listed update date is 2025-09-15. For exact protocol behavior, use the cited specification rather than interpreting a short label as a complete instruction.
1xx: informational
| Code | Registered name | Practical reading |
|---|---|---|
| 100 | Continue | The client may continue sending the request. |
| 101 | Switching Protocols | The protocol is switching. |
| 102 | Processing | Registered extension associated with WebDAV. |
| 103 | Early Hints | Provides early response hints. |
| 104 | Upload Resumption Supported | Temporary registration. IANA listed it as registered 2024-11-13, with an extension registration dated 2025-09-15 and expiry 2026-11-13; check the registry for current status. |
2xx: success
| Code | Registered name | Practical reading |
|---|---|---|
| 200 | OK | General successful response. |
| 201 | Created | Indicates creation. |
| 202 | Accepted | Indicates acceptance; consult the API’s documentation for what happens next. |
| 203 | Non-Authoritative Information | A distinct success status; refer to HTTP semantics for its precise use. |
| 204 | No Content | Successful response with no content. |
| 205 | Reset Content | Signals reset content semantics. |
| 206 | Partial Content | Indicates a partial response. |
| 207 | Multi-Status | Registered extension code. |
| 208 | Already Reported | Registered extension code. |
| 226 | IM Used | Registered extension code. |
3xx: redirection
| Code | Registered name | Practical reading |
|---|---|---|
| 300 | Multiple Choices | Multiple choices are available. |
| 301 | Moved Permanently | Permanent redirect status; do not assume it behaves exactly like other redirect codes. |
| 302 | Found | Redirect status with semantics distinct from 301, 303, 307, and 308. |
| 303 | See Other | A distinct redirect status; consult RFC 9110 for request handling. |
| 304 | Not Modified | Registered redirect-class status; consult HTTP semantics for its use with conditional requests and caching. |
| 305 | Use Proxy | Registered but marked unused by the current HTTP semantics reference. |
| 306 | (Unused) | Unused value. |
| 307 | Temporary Redirect | Temporary redirect with semantics distinct from 302 and 303. |
| 308 | Permanent Redirect | Permanent redirect with semantics distinct from 301 and 303. |
4xx: client error
| Code | Registered name | Practical reading |
|---|---|---|
| 400 | Bad Request | The request is malformed or otherwise cannot be handled as sent. |
| 401 | Unauthorized | Authentication-related status; see the distinction below. |
| 402 | Payment Required | Registered status; the name alone does not establish how a particular API uses it. |
| 403 | Forbidden | Refusal status; see the distinction below. |
| 404 | Not Found | The requested resource was not found, or the server is not willing to disclose that it exists. |
| 405 | Method Not Allowed | The method is not allowed for the target resource. |
| 406 | Not Acceptable | The response cannot meet the request’s stated content-negotiation preferences. |
| 407 | Proxy Authentication Required | Authentication is required by a proxy. |
| 408 | Request Timeout | The request timed out. |
| 409 | Conflict | The request conflicts with the current state of the resource. |
| 410 | Gone | The resource is no longer available and that condition is understood to be likely permanent. |
| 411 | Length Required | The server requires a defined request length. |
| 412 | Precondition Failed | A request precondition was not met. |
| 413 | Content Too Large | The request content is too large for the server to process. |
| 414 | URI Too Long | The request URI is too long for the server to process. |
| 415 | Unsupported Media Type | The request’s media type is not supported for the method or resource. |
| 416 | Range Not Satisfiable | The requested range cannot be satisfied. |
| 417 | Expectation Failed | A request expectation could not be met. |
| 418 | (Unused) | Unused value. |
| 421 | Misdirected Request | The request was directed to a server that cannot produce a response. |
| 422 | Unprocessable Content | The content type and syntax are understood, but the contained instructions cannot be processed. |
| 423 | Locked | Registered extension code. |
| 424 | Failed Dependency | Registered extension code. |
| 425 | Too Early | The server is unwilling to risk processing a request that might be replayed. |
| 426 | Upgrade Required | The client needs to switch to a different protocol. |
| 428 | Precondition Required | The server requires the request to be conditional. |
| 429 | Too Many Requests | The client has sent too many requests in a given time; see retry guidance below. |
| 431 | Request Header Fields Too Large | The server will not process the request because its header fields are too large. |
| 451 | Unavailable For Legal Reasons | The resource is unavailable for legal reasons. |
5xx: server error
| Code | Registered name | Practical reading |
|---|---|---|
| 500 | Internal Server Error | General server failure; the code does not reveal its cause. |
| 501 | Not Implemented | The server does not support the functionality required to fulfill the request. |
| 502 | Bad Gateway | A gateway or proxy received an invalid response from an upstream server. |
| 503 | Service Unavailable | The server is temporarily unable to handle the request. |
| 504 | Gateway Timeout | A gateway or proxy did not receive a timely response from an upstream server. |
| 505 | HTTP Version Not Supported | The server does not support the HTTP version used in the request. |
| 506 | Variant Also Negotiates | Registered extension code. |
| 507 | Insufficient Storage | Registered extension code. |
| 508 | Loop Detected | Registered extension code. |
| 510 | Not Extended | Registered but marked obsolete. |
| 511 | Network Authentication Required | Authentication is required to access the network. |
How to distinguish status codes that look similar
401 Unauthorized vs. 403 Forbidden
Despite its name, 401 is the authentication challenge status: under RFC 9110, a request lacks valid authentication credentials for the target resource, and the response must include a WWW-Authenticate challenge. 403 means the server understood the request but refuses to fulfill it; valid credentials do not necessarily change that refusal. Check the authentication scheme and challenge for 401, and the resource’s access policy for 403. Do not treat either code as a universal instruction to log in or change permissions.
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404 Not Found vs. 410 Gone
404 says the server did not find a current representation of the target resource, or is unwilling to disclose that one exists. 410 is for a condition understood to be likely permanent: the resource is no longer available and the server has no forwarding address. The distinction is about the server’s knowledge and resource state, not merely whether a user can find a page.
301, 302, 303, 307, and 308 redirects
These are not interchangeable “redirect” labels. 301 and 308 express permanent redirection; 302 and 307 express temporary redirection; 303 directs the client to retrieve another resource. RFC 9110 distinguishes their handling of the request method: 307 and 308 preserve it, while 303 changes the follow-up to retrieval with GET or HEAD; historical behavior for 301 and 302 can change POST to GET. Clients and servers should follow RFC 9110 and their documented compatibility requirements rather than assuming all redirects preserve a request body or method.
429 Too Many Requests vs. 503 Service Unavailable
429 identifies rate limiting; 503 indicates temporary service unavailability. Neither code by itself tells a client exactly when a retry is safe. Check response headers such as Retry-After, the API’s rate-limit documentation, and whether repeating the operation is safe before retrying. A client should avoid an immediate retry loop.
422 Unprocessable Content
The current registered name is “Unprocessable Content.” Older material may call it “Unprocessable Entity”; that is legacy wording for the status now registered as 422. It describes content that the server can understand syntactically but cannot process as instructed.
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How to use a status code when troubleshooting
- Read the exact three-digit code. The first digit gives the broad class; the full code is the useful starting point.
- Check the response details. Inspect headers and any response body for authentication challenges, retry timing, redirect targets, or application-specific error information.
- Compare with the request. Verify the method, URL, headers, content type, body, credentials, and any conditional or range headers relevant to the code.
- Use the applicable specification and API documentation. IANA provides the registered names and references; RFC 9110 defines core HTTP semantics, while extension codes may refer to other RFCs.
- For a 5xx, trace the responding layer. A proxy, gateway, application server, or upstream service can produce a server-class response; correlate server logs and request identifiers instead of assuming the origin application is at fault.
Registered does not mean currently usable
The IANA registry is the authoritative inventory of registered HTTP status-code names and their references. Its listed update date is 2025-09-15. It includes core codes and extensions, as well as values marked temporary, unused, or obsolete. It also contains unassigned ranges: an unassigned number has no registered meaning, so do not infer one from its position between assigned codes. The 104 registration was temporary through the listed expiry date of 2026-11-13; consult the live registry for any updated status.
For implementation decisions, follow the specification linked from the relevant registry entry. The registry is an inventory, not a substitute for normative rules about method handling, caching, authentication, or retries.
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