Each extra "nine" of uptime — 99% → 99.9% → 99.99% — costs roughly an order of magnitude more effort.
99% allows 3.7 days of downtime a year; 99.99% allows 52 minutes. Chasing nines past what users notice is expensive vanity.
Not below the level your dependencies can deliver. Four nines of application uptime on a three-nines cloud region is unreachable at any price, so the cost curve does not rise — it goes vertical. And the first nines are often nearly free, so the multiplier only describes the middle.
Estimate with the rule, then check it against the calculator that models it properly.
Open Uptime & SLA Calculator →