503 Service Unavailable means the server cannot handle the request right now because of a temporary overload or scheduled maintenance, which will likely be resolved after some delay. The server may send a Retry-After header with the number of seconds, or a date, after which the client should try again.
Servers do not have to answer with 503 when they are overloaded; some simply refuse connections. Applications return 503 during deployments or maintenance, load balancers when no healthy backend is available, and some rate limiters use it instead of 429.
Example response
HTTP/1.1 503 Service Unavailable
Date: Fri, 06 Nov 2026 10:15:32 GMT
Retry-After: 3600
Cache-Control: no-store
Content-Type: text/html; charset=utf-8
Content-Length: 125
<!doctype html>
<title>Down for maintenance</title>
<h1>We'll be back soon</h1>
<p>Scheduled maintenance until 11:15 UTC.</p>SEO and redirect checking
503 is the correct status for planned maintenance. Google treats it as temporary: Googlebot backs off and tries again later, and indexed pages are kept as long as the outage is short. Serving a maintenance page with 200 risks replacing your content in the index, and serving 404 risks having pages removed. Send Retry-After and keep the downtime short; if a site keeps returning 503 for more than a few days, Google may start dropping URLs.
If robots.txt also returns 503, Google pauses crawling of the whole site, which is usually fine for a short maintenance window.
Common causes and use cases
- Scheduled maintenance or a deployment, such as WordPress's "Briefly unavailable for scheduled maintenance" page.
- A traffic spike overloads the application or exhausts its worker pool.
- A load balancer has no healthy backend servers.
- The hosting account hit its resource limits or was suspended.