HTTP Status Codes Explained (100–599) + Common Errors

Every time a browser requests a webpage, the web server responds with an HTTP status code. These codes tell browsers, search engines, APIs, and monitoring tools whether the request succeeded, failed, or needs additional action.

Understanding HTTP status codes helps website owners quickly identify problems that affect uptime, user experience, and SEO.

What Is an HTTP Status Code?

An HTTP status code is a three-digit number returned by a web server after receiving a request.

For example:

• Browser → https://example.com
• Server → 200 OK

Or:

• Browser → https://example.com/missing-page
• Server → 404 Not Found

These responses tell browsers exactly what happened.

HTTP Status Code Categories

There are five main categories:

1xx Informational Codes

These indicate the request has been received and processing continues.

100 Continue

The server received the initial request and expects the client to continue sending data. Common in API communications.

101 Switching Protocols

The server agrees to switch protocols. Example: HTTP → WebSocket

2xx Success Codes

These indicate everything worked correctly.

200 OK

The most common response. Everything worked as expected. Pages loaded successfully, APIs returned data, search engines can crawl the page.

201 Created

Used after creating a resource (usually in APIs). A new user or item was successfully created.

202 Accepted

The request has been accepted but is still processing. Often used for background jobs.

204 No Content

The request succeeded but there is nothing to return. Common for DELETE requests.

3xx Redirect Codes

These tell browsers to visit another URL.

301 Moved Permanently

The page permanently moved. Good for SEO because search engines transfer ranking signals to the new URL.

302 Found

Temporary redirect. Use only when the page will return.

304 Not Modified

The browser already has the latest cached version. Speeds up websites by reducing downloads.

307 & 308

Temporary and permanent redirects that preserve the original HTTP method.

4xx Client Errors

These indicate a problem with the request.

400 Bad Request

The request is invalid. Possible causes: invalid syntax, corrupted request, or missing parameters.

401 Unauthorized

Authentication is required. The user must log in before accessing the resource.

403 Forbidden

The server understands the request but refuses access. Common causes: IP blocked, file permissions, or security rules.

404 Not Found

One of the most common errors. The requested page does not exist.

Common reasons:

• Deleted page
• Broken links
• Incorrect URL
• Typo

SEO Impact: Too many 404 errors can hurt user experience. If a page has permanently moved, use a 301 redirect instead of leaving a broken link.

405 Method Not Allowed

The server received the request but the HTTP method (GET, POST, etc.) is not allowed for that resource.

408 Request Timeout

The client took too long to send the request.

429 Too Many Requests

The client exceeded the allowed request rate. Usually caused by rate limiting, bots, or excessive API requests.

5xx Server Errors

These indicate the problem is on the server, not the client.

500 Internal Server Error

The most common server error. Possible causes: PHP errors, plugin failures, database issues, or configuration problems.

Action: If your monitoring service detects repeated 500 responses, investigate server logs immediately.

501 Not Implemented

The server does not support the requested functionality.

502 Bad Gateway

Occurs when one server receives an invalid response from another. Common with reverse proxies, CDNs, and load balancers.

503 Service Unavailable

The server is temporarily unavailable. Common causes: maintenance, server overload, high traffic, or resource exhaustion.

Note: Unlike a 500 error, a 503 often indicates a temporary condition.

504 Gateway Timeout

One server waited too long for another server to respond. Common causes: slow backend, database delays, or network issues.

HTTP Status Codes and SEO

Search engines use status codes to understand your website:

• 200 OK - Page can be indexed
• 301 - Ranking signals are transferred to the new URL
• 302 - Usually treated as temporary (no ranking transfer)
• 404 - Page may eventually be removed from the search index
• 500 - Persistent server errors can prevent pages from being crawled and indexed

Monitoring these responses helps maintain search visibility.

Why Website Monitoring Matters

Checking a homepage isn't enough. A monitoring system should detect:

• 500 Internal Server Errors
• 502 Bad Gateway
• 503 Service Unavailable
• 504 Gateway Timeout
• Redirect loops
• Unexpected 404 responses
• Slow response times

Receiving alerts quickly can reduce downtime and improve the experience for visitors.

Best Practices

• Keep important pages returning 200 OK
• Use 301 redirects when URLs permanently change
• Fix broken internal links to reduce 404 errors
• Investigate recurring 5xx errors promptly
• Monitor response times as well as status codes
• Review server logs when unexpected errors appear

Frequently Asked Questions

What is the most common HTTP status code?

200 OK, indicating the request was successful.

Is a 404 error bad?

An occasional 404 is normal, but important pages should either exist or permanently redirect if they've moved.

What's the difference between 500 and 503?

A 500 Internal Server Error indicates an unexpected server-side failure, while a 503 Service Unavailable usually means the server is temporarily unable to handle requests, such as during maintenance or heavy load.

Does a 301 redirect help SEO?

Yes. A properly implemented 301 redirect tells search engines that a page has permanently moved and helps preserve ranking signals.

How can I monitor HTTP status codes?

Use a website monitoring service that checks your URLs regularly and alerts you when they return unexpected responses like 404, 500, 502, or 503, so issues can be addressed quickly.