SLA Uptime Explained: How Much Downtime Does 99.9% Really Allow?
An uptime SLA sounds reassuring until you convert it to real downtime. Here's what each 'nine' means in minutes and hours, and how to read an SLA.
What an uptime SLA means
A service level agreement (SLA) often promises a percentage of uptime. The headline number hides what really matters: how much downtime it permits. The free SLA Uptime Calculator converts any target into allowed downtime per day, month, and year.
The maths of nines
Allowed downtime = (1 − uptime% ÷ 100) × the time period. Each additional nine cuts permitted downtime by roughly ten times:
- 99% ("two nines") — about 7.2 hours of downtime per month.
- 99.9% ("three nines") — about 43.8 minutes per month.
- 99.99% ("four nines") — about 4.4 minutes per month.
Reading an SLA critically
Check what the SLA actually covers — scheduled maintenance is often excluded, and the remedy for a breach is usually a service credit, not compensation for your losses. Match the SLA tier to how critical the service is to you.
Pair with support metrics
Uptime is one half of reliability; responsiveness is the other. Track it with the CSAT and Support Cost calculators.
Use the free SLA Uptime Calculator | Support Cost Calculator