503 Service Unavailable
5xx Server error
The server cannot take requests right now — maintenance or overload.
| On the wire | HTTP/1.1 503 Service Unavailable |
|---|---|
| Code | 503 |
| Is it an error | Yes — this is an error |
| Category | 5xx Server error |
The problem is on the server side; retrying later is often worth it.
How to read this
- The first digit of a status code carries the meaning: 4 means the request, 5 means the server.
- 404 means "not there"; 403 means "there, but not for you". Swapping them makes debugging slower.
- Header names are case-insensitive — Content-Type and content-type are the same header.
- Caching and CORS problems usually come down to one header line, visible in the browser’s network tab.
Frequently asked questions
Q. What does HTTP 503 Service Unavailable mean?
The server cannot take requests right now — maintenance or overload.
Q. What does 503 Service Unavailable look like on the wire?
It travels as HTTP/1.1 503 Service Unavailable
Q. Is 503 Service Unavailable an error?
Yes. A code starting with 4 blames the request; one starting with 5 blames the server.
Q. What group does 503 Service Unavailable belong to?
The 5xx server error group; the rest of that group is listed further down.