HTTP Status Codes — Complete Reference
Every HTTP status code from 1xx to 5xx with descriptions. Search by code number, name, or keyword. Filter by category to quickly find what you need.
1 result
5xx Server Error
Service Unavailable
The server is not ready to handle the request — commonly due to maintenance or overloading.
HTTP Status Code Categories
HTTP status codes are grouped into five classes based on the first digit of the response code:
| Range | Category | Meaning |
|---|---|---|
| 1xx | Informational | Request received and understood, processing continues |
| 2xx | Success | Request was successfully received, understood, and accepted |
| 3xx | Redirection | Further action must be taken to complete the request |
| 4xx | Client Error | The request contains bad syntax or cannot be fulfilled |
| 5xx | Server Error | The server failed to fulfil an apparently valid request |
Most Common HTTP Status Codes
- 200 OK — The standard success response for GET, POST, and PUT requests.
- 201 Created — Returned after a successful POST that creates a new resource.
- 204 No Content — Successful request with no response body (common for DELETE).
- 301 Moved Permanently — SEO-friendly permanent redirect.
- 304 Not Modified — Browser cache is fresh; no new data sent.
- 400 Bad Request — Client sent malformed or invalid data.
- 401 Unauthorized — Authentication required.
- 403 Forbidden — Authenticated but not authorized.
- 404 Not Found — Resource does not exist at this URL.
- 422 Unprocessable Content — Validation failed (common in REST APIs).
- 429 Too Many Requests — Rate limit exceeded.
- 500 Internal Server Error — Generic server-side error.
- 502 Bad Gateway — Upstream server returned an invalid response.
- 503 Service Unavailable — Server down or overloaded.
Frequently Asked Questions
What is the difference between 401 and 403?
401 Unauthorized means the request lacks valid authentication credentials — the server doesn't know who you are. 403 Forbidden means authentication succeeded but the authenticated user does not have permission to access the resource — the server knows who you are but says no.
What is the difference between 301 and 302?
301 Moved Permanently tells browsers and search engines that the resource has moved forever — the new URL should replace the old one in bookmarks and search indexes. 302 Found is a temporary redirect — the old URL should be kept. For SEO, always use 301 for permanent moves.
What does 422 mean in a REST API?
422 Unprocessable Content (formerly "Unprocessable Entity") means the request body is syntactically correct but contains semantic errors — such as a validation failure (e.g. a required field is missing or a value is out of range). It's the preferred response for API validation errors, compared to the more generic 400.
When should I use 503 vs 500?
Use 500 Internal Server Error for unexpected bugs or unhandled exceptions. Use 503 Service Unavailable when the server is intentionally down (maintenance mode) or temporarily overloaded and requests should be retried later. Pair 503 with a Retry-After header to tell clients when to try again.