Rickard (2021) makes it clear that adding 9's to your SLAs is a big commitment. While the meaning of a 9 scales linearly, the effort goes exponentially.
Anyways, for refs:
Uptime - Downtime (Yearly)
99.00000% - 3d 15h 39m
99.90000% - 8h 45m 56s
99.99000% - 52m 35s
99.99900% - 5m 15s
99.99990% - 31s
99.99999% - 3sThese numbers can be quite useful with napkin math numbers. It's quite handy to have these at hand.
2026-10: "The meaning of a 9 scales linearly" isn't quite right. Each extra 9 cuts your downtime budget by 10x. It only looks linear because you're adding one digit. Which is exactly why the effort explodes.