HTTP status code
An HTTP status code is a three-digit numerical response issued by a web server indicating the outcome of a client request. Search engines interpret status codes (such as 200 for success, 301 for permanent redirects, 404 for missing pages, and 500 for server errors) to determine whether a URL should be crawled and indexed.
In plain English
Whenever a web browser or search engine crawler requests a webpage from a web server, the server delivers an initial header response before transmitting any visual content or HTML code. The very first line of that header contains a standardized three-digit number called an HTTP status code.
These codes form a universal communication language between clients and servers. They inform the requester whether the page was found successfully, whether it has permanently moved to a new address, whether authentication is required, or whether the server suffered an internal crash. Search engines evaluate status codes as their first line of inquiry; the returned code determines whether a crawler should download the page, update its index, follow a redirect, or drop the URL from search results entirely.
An example
HTTP status codes are categorized into five distinct classes, each carrying profound consequences for search engine crawlers:
200 OK: The request succeeded. The crawler downloads the HTML document and passes it along to the rendering and indexing pipeline.301 Moved Permanently: The requested URL has moved to a new destination permanently. The crawler updates its index to reference the new URL and transfers accumulated link equity from the old address to the new one.302 Found(Temporary Redirect): The page is temporarily located at an alternate address. The crawler follows the redirect to fetch content, but retains the original URL in the index because the move is not permanent.404 Not Found: The server cannot find the requested resource. The crawler records the missing document and eventually removes the URL from the search index.410 Gone: The page was intentionally and permanently deleted. Search engines drop 410 URLs from their indexes much faster than standard 404s.500 Internal Server Error: The origin web server experienced an internal failure while trying to process the request. When crawlers receive 500-level error codes, they immediately reduce their crawl rate to avoid crashing the server.
A dangerous technical mistake is a soft 404 error. A soft 404 occurs when a server renders a “Sorry, this page does not exist” message visually to human readers, but erroneously returns an HTTP 200 OK header code. This tricks search engines into indexing thousands of useless error pages, squandering crawl capacity and cluttering search results.
Why it matters
HTTP status codes govern how search engines discover, crawl, and preserve the authority of your web pages. Delivering correct status codes ensures smooth site migrations, prevents broken links from lingering in search results, and protects crawl budget. Regularly auditing status codes in your server access logs is one of the most effective ways to maintain technical website health.
Related terms
Read the full guide to technical SEO, or explore our complete guide to HTTP status codes.