It depends on the infrastructure you're running on. There was a post yesterday going fairly into depth how you do such calculations https://authress.io/knowledge-base/articles/2025/11/01/how-w...

You probably cannot achieve this with a single node, so you'll at least need to replicate it a few times to combat the normal 2-3 9s you get from a single node. But then you've got load balancers and dns, which can also serve as single point of failure, as seen with cloudflare.

Depending on the database type and choice, it varies. If you've got a single node of postgres, you can likely never achieve more than 2-3 9s (aws guarantees 3 9s for a multi-az RDS). But if you do multi-master cockroach etc, you can maybe achieve 5 9s just on the database layer, or using spanner. But you'll basically need to have 5 9s which means quite a bit of redundancy in all the layers going to and from your app and data. The database and DNS being the most difficult.

Reliable DNS provider with 5 9s of uptime guarantees -> multi-master load balancer each with 3 9s, -> each load balancer serving 3 or more apps each with 3 9s of availability, going to a database(s) with 5 9s.

This page from google shows their uptime guarantees for big tables, 3 9s for a single region with a cluster. 4 9s for multi cluster and 5 9s for multi region

https://docs.cloud.google.com/architecture/infra-reliability...

In general it doesn't matter really what you're running, it is all about redundancy. Whether that is instances, cloud vendor, region, zone etc.