What are HTTP status codes?
An HTTP status code is the three-digit number a server puts at the start of every response to say how the request went. The first digit is the class: 1xx is informational, 2xx succeeded, 3xx means go somewhere else, 4xx blames the request, and 5xx admits the server broke. The distinctions that matter in practice are the narrow ones — 401 versus 403, 301 versus 302, 400 versus 422 — because getting them wrong changes how browsers cache, how search engines treat your URLs, and whether a client knows it is worth retrying.
How to use this tool
- 1 Type a number or a word into the search box — "502", "timeout" and "redirect" all match.
- 2 Use the class filters to narrow to just 4xx client errors or 5xx server errors.
- 3 Read the meaning column for the practical distinction, not just the spec name.
- 4 Every code is in the page source, so browser find (Ctrl+F / Cmd+F) works too.
Frequently asked questions
What's the difference between 401 and 403?
401 means unauthenticated — you sent no credentials or bad ones, and retrying with valid credentials will work. It must include a WWW-Authenticate header. 403 means authenticated but not permitted; the same credentials will never work, so there is no point retrying. Sending 401 when you mean 403 makes clients loop on a login prompt forever.
Should I use 301 or 302 for a redirect?
301 is permanent: search engines transfer ranking signals to the target and browsers cache it aggressively — sometimes indefinitely, which makes a mistaken 301 painful to undo. 302 is temporary and keeps the original URL canonical. Use 301 only when the old URL is genuinely retired for good.
When should I return 422 instead of 400?
400 means the server could not parse the request at all — malformed JSON, a broken header. 422 means the syntax parsed fine but the content failed your validation rules, like a missing required field or an email that is not an email. Most API validation errors are 422, not 400.
Do 404s hurt my SEO?
No. Google has stated repeatedly that 404s are a normal part of the web and do not penalise a site. What hurts is a soft 404 — returning 200 for a page that does not exist — because it fills the index with duplicate content. If a page is deliberately and permanently gone, 410 tells Google that explicitly and deindexes it faster.
What does 502 Bad Gateway actually mean?
A proxy or load balancer forwarded your request upstream and got a reply it could not understand — or no reply at all. Almost always it means the application behind the proxy has crashed, is not listening on the expected port, or was killed mid-response. It is a problem behind the proxy, not in the proxy itself.