An availability figure translates into allowed annual downtime:
- 99%: ~3.65 days/year — unacceptable in production.
- 99.9% (“three nines”): ~8h45/year.
- 99.99% (“four nines”): ~53 minutes/year — the standard of good data centers.
- 99.999% (“five nines”): ~5 minutes/year — critical infrastructure.
But the figure alone is a trap. What matters is the scope (does the SLA cover power, cooling, network, or only one of the three?), the calculation basis (monthly or annual), the exclusion of planned maintenance, and above all the penalties actually owed on a breach (often a capped credit, nowhere near your business loss).
Availability correlates with — but is not the same as — the site’s Tier level, and is complemented on the network side by the repair SLA of your access links.