1xx Informational

100 Continue - Client should continue with request
101 Switching Protocols - Server switching to different protocol
102 Processing - Server received and is processing (WebDAV)
103 Early Hints - Preload resources while server prepares response

2xx Success

200 OK - Request succeeded
201 Created - New resource created (POST/PUT)
202 Accepted - Request accepted but not yet processed
203 Non-Authoritative Info - Modified by proxy
204 No Content - Success but no body returned
205 Reset Content - Reset document view
206 Partial Content - Range request fulfilled
207 Multi-Status - Multiple status codes (WebDAV)
208 Already Reported - DAV binding already reported
226 IM Used - Delta encoding applied

3xx Redirection

300 Multiple Choices - Multiple options for resource
301 Moved Permanently - Resource permanently moved (cacheable)
302 Found - Temporary redirect (historically "Moved Temporarily")
303 See Other - Redirect with GET method
304 Not Modified - Cached version still valid
305 Use Proxy - Deprecated, use proxy
307 Temporary Redirect - Temporary, keep original method
308 Permanent Redirect - Permanent, keep original method

4xx Client Errors

400 Bad Request - Malformed request syntax
401 Unauthorized - Authentication required
402 Payment Required - Reserved for future use
403 Forbidden - Server refuses to authorize
404 Not Found - Resource does not exist
405 Method Not Allowed - HTTP method not supported
406 Not Acceptable - No content matches Accept headers
407 Proxy Auth Required - Authenticate with proxy
408 Request Timeout - Client took too long
409 Conflict - Request conflicts with current state
410 Gone - Resource permanently deleted
411 Length Required - Content-Length header required
412 Precondition Failed - Conditional request failed
413 Payload Too Large - Request body too large
414 URI Too Long - URL exceeds server limits
415 Unsupported Media Type - Content-Type not supported
416 Range Not Satisfiable - Invalid Range header
417 Expectation Failed - Expect header cannot be met
418 I'm a Teapot - Easter egg (RFC 2324)
421 Misdirected Request - Wrong server for request
422 Unprocessable Entity - Semantic errors (WebDAV)
423 Locked - Resource is locked (WebDAV)
424 Failed Dependency - Depends on failed request (WebDAV)
425 Too Early - Server unwilling to risk replay
426 Upgrade Required - Client should switch protocols
428 Precondition Required - Conditional request needed
429 Too Many Requests - Rate limit exceeded
431 Request Header Fields Too Large - Headers too big
451 Unavailable For Legal Reasons - Censored/blocked

5xx Server Errors

500 Internal Server Error - Generic server error
501 Not Implemented - Method not supported
502 Bad Gateway - Invalid response from upstream
503 Service Unavailable - Server overloaded/maintenance
504 Gateway Timeout - Upstream server timed out
505 HTTP Version Not Supported - HTTP version not supported
506 Variant Also Negotiates - Circular reference
507 Insufficient Storage - Server storage full (WebDAV)
508 Loop Detected - Infinite loop detected (WebDAV)
510 Not Extended - Extensions required
511 Network Auth Required - Captive portal login needed

Common Non-Standard Codes

420 Enhance Your Calm - Twitter rate limit (deprecated)
444 No Response - Nginx closed connection silently
499 Client Closed Request - Nginx, client disconnected
520 Web Server Error - Cloudflare unknown error
521 Web Server Down - Cloudflare cannot reach origin
522 Connection Timed Out - Cloudflare timeout
523 Origin Unreachable - Cloudflare DNS error
524 Timeout Occurred - Cloudflare origin timeout
525 SSL Handshake Failed - Cloudflare SSL error
526 Invalid SSL Certificate - Cloudflare bad cert
530 Origin DNS Error - Cloudflare DNS lookup failed

Quick Reference

1xx Informational
2xx Success
3xx Redirection
4xx Client Error
5xx Server Error