HTTP status codes a site owner needs to know

A status code is a three-digit number a web server sends back with every page it serves. Your browser reads it before it shows you anything. You never see it, until something goes wrong.

Don't try to learn them all. Six are enough for a site owner.

The six that matter

200: all good, mostly

A 200 means the request succeeded.¹ That's what you want for every real page.

Here's the trap. A page can return a 200 and still be useless, like an empty page or a "sorry, not found" message that visitors read as an error. Google's help says a 2xx code doesn't guarantee a page will be indexed, either.⁷

301 and 302: sending people elsewhere

Both are redirects. The server tells the browser to load a different address, and the browser does it on its own. For a 301, the page has been permanently moved to the new address.² For a 302, it has been temporarily moved.³

Which one you pick changes how search engines treat the move. Google treats a 301 as a strong signal and a 302 as a weak one.⁷ The redirect comparison goes through it.

One more point about redirects. By default Google's crawlers follow up to 10 hops in a row.⁷ Don't send people on a long trip when one hop will do.

404 and 410: the page isn't there

A 404 says the server can't find the page.⁴ It doesn't say whether that's for a day or forever. If you're not sure, MDN says a 404 is the right choice.⁵

A 410 is firmer. It says the page is no longer available and the condition is likely to be permanent.⁵ MDN says a permanently removed page should send a 410 rather than a 404.⁴

Mistyped addresses and unredirected moves are common causes.⁴ Why missing pages happen has more.

What do search engines do with them? Google doesn't use the content from addresses that return 4xx codes.⁷

500: it's your server, not the address

A 500 means the server hit an unexpected condition and couldn't finish the request.⁶ It's a catch-all. The server knows something's wrong but can't say what.

Causes include improper server configuration, among others.⁶ Visitors can't fix it. MDN says these issues need investigation by server owners or administrators.⁶

Think back to what you changed just before the errors began, such as a new redirect rule. Undo it if you can. If that doesn't help, ask your host's support to check the server's error log.

How to see a page's status code

Link-checking tools and some hosting dashboards show it. The method depends on the tool, so check its help pages. Look. A page can seem fine and still send the wrong code.