How to use the HTTP Status Codes
- Type a code such as 502, a class such as 4xx, or a keyword such as timeout or cookies.
- Use the class buttons to show only informational, success, redirect, client error or server error codes.
- Click any code to expand its meaning. Common errors include their typical causes and how to fix them.
- Untick Include unofficial codes to hide server-specific codes from nginx, Cloudflare, IIS and others.
Features
- Every standard code from RFC 9110 and related RFCs
- Common unofficial codes from nginx, Cloudflare and IIS, clearly marked
- Causes and fixes for the errors you meet most often
- Instant search by number, class or keyword
- Filter by class with one click
- Direct links to any code, such as #code-404
The five classes of status codes
Every HTTP response starts with a three-digit status code that tells the client how the request went. The first digit defines the class, so even an unfamiliar code can be understood at a glance. The current definitions are in RFC 9110, published in 2022, with a few extra codes defined in separate RFCs and registered with IANA.
| Class | Meaning | Who needs to act |
|---|---|---|
| 1xx | Informational: the request is in progress | Nobody; handled by software |
| 2xx | Success: the request worked | Nobody |
| 3xx | Redirection: look somewhere else | The client follows the new URL |
| 4xx | Client error: the request was wrong | Whoever sent the request |
| 5xx | Server error: the server failed | The site owner or operator |
Clients that meet a code they do not recognize treat it like the x00 code of its class, so an unknown 4xx is handled as 400 and an unknown 5xx as 500. That is why server-specific codes such as nginx’s 499 or Cloudflare’s 52x can exist without breaking browsers.
How status codes affect SEO
301 and 308 versus 302 and 307
Permanent redirects (301 and 308) tell search engines that the new URL replaces the old one, so the new address is indexed and ranking signals are consolidated there. Temporary redirects (302 and 307) tell them the original URL is still the one to keep. Google has said it may eventually treat a long-standing temporary redirect as permanent, but relying on that wastes time. When you move a page for good, use 301 or 308, point each old URL at its closest equivalent, and keep redirect chains to a single hop.
404 versus 410
Both remove a URL from search results over time. A 410 says the removal is deliberate and final, which can lead to slightly faster removal, while 404 leaves the door open. Neither hurts the rest of your site: missing pages are a normal part of the web. What does hurt is a soft 404, a “page not found” message served with a 200 status. Search engines may index those empty pages or flag them as errors, so make sure missing content returns a real 404 or 410.
5xx errors and crawling
When a site returns server errors or 429 responses, search engine crawlers slow down to avoid overloading it. If errors persist for days, affected URLs can be dropped from the index. For planned maintenance, return 503 Service Unavailable with a Retry-After header rather than a 200 maintenance page or a 404, which signals that the outage is temporary.
Checking the status code of a page
In any desktop browser, open the developer tools (F12), switch to the Network tab and reload: the Status column shows the code for every request, including redirects. From a terminal, curl -I https://example.com/page prints just the response headers, and adding -L follows redirects so you can see each hop in a chain.
Mistakes to avoid
- Returning 200 for errors in an API, which forces clients to parse the body to find out something failed.
- Using 403 when the user simply is not logged in; that is what 401 is for.
- Redirecting every removed page to the home page, which search engines usually treat as a soft 404 anyway.
- Hiding real server errors behind a generic 500 page without logging the cause.
Frequently asked questions
What is the difference between 401 and 403?
401 means you are not authenticated: log in or send valid credentials and try again. 403 means the server knows who you are but you do not have permission, so logging in again will not help.
Is a 301 or a 302 redirect better for SEO?
Use 301 (or 308) when a page has moved permanently, so search engines index the new URL. Use 302 (or 307) only for temporary moves where the original URL should stay indexed.
What causes a 502 Bad Gateway error?
A proxy, load balancer or CDN could not get a valid response from the application server behind it, usually because that service crashed, is not running or is misconfigured.
Do 404 errors hurt my rankings?
Not by themselves. Missing pages are normal. Fix broken internal links, redirect pages that moved, and make sure missing pages return a real 404 instead of a 200 page.
What are unofficial status codes like 499 or 522?
They are codes invented by specific servers or services, such as nginx and Cloudflare, to report situations the standards do not cover. They are marked Unofficial in this list.