Question 1

What does 99.9% uptime mean in actual downtime?

Accepted Answer

About 8 hours and 45 minutes per year, or 43 minutes per month. This is the standard target for most business applications.

Question 2

What is the difference between three nines and four nines?

Accepted Answer

Each additional nine is a 10x reduction in allowed downtime. Three nines allows 8.76 hours per year, while four nines allows only 52.6 minutes.

Question 3

How is SLA downtime calculated?

Accepted Answer

Subtract uptime from 100% and apply that ratio to your time period. For 99.9% uptime over a year, that equals about 8.76 hours of allowed downtime.

Question 4

What is considered a good SLA uptime percentage?

Accepted Answer

99.9% is standard for most business apps, 99.99% for critical infrastructure, and 99.999% for mission-critical systems.

Question 5

Does scheduled maintenance count against SLA uptime?

Accepted Answer

It depends on the agreement. Many providers exclude scheduled maintenance if proper notice is given. Always check the fine print.

Question 6

What happens if a provider fails to meet their SLA?

Accepted Answer

Most SLAs include service credits when targets are missed. The compensation scales with severity, typically ranging from 10% to 30% credits.

Uptime	Name	Downtime/Month	Downtime/Year
99%	Two Nines	7h 12m	3d 15h 36m
99.5%	Two and a Half Nines	3h 36m	1d 19h 48m
99.9%	Three Nines	43m 11s	8h 45m 35s
99.95%	Three and a Half Nines	21m 35s	4h 22m 47s
99.99%	Four Nines	4m 19s	52m 33s
99.999%	Five Nines	25s	5m 15s

SLA Uptime Calculator

99.9% Uptime