Service Unavailable

The server is down for maintenance or overloaded. Try again later.

503 Service Unavailable means the server can't handle the request right now - it's overloaded, restarting, or deliberately down for maintenance. It's meant to be temporary, and a good 503 includes a Retry-After header. Unlike 500, the server is working as designed; it just can't serve you at this moment.

What 503 looks like on the wire

GET / HTTP/1.1
Host: example.com

HTTP/1.1 503 Service Unavailable
Retry-After: 120
Content-Type: text/html

About 5xx Server Error

The server failed to fulfill a valid request. The problem is on the server side, not the client.

How to fix 503 Service Unavailable

If it's your server, check load, memory, and worker/process limits.
Confirm it isn't stuck in a deploy or maintenance window.
Scale up or out if traffic is genuinely exceeding capacity.
As a client, back off and retry per the Retry-After header.

Where it's defined

503 Service Unavailable is defined in RFC 9110 §15.6.4. MDN's 503 reference covers browser behavior and examples.