How to use the Server Uptime SLA
- Enter the uptime percentage you are promising or evaluating.
- Read the permitted downtime per day, week, month and year.
- Check whether maintenance windows are excluded in the agreement.
- Compare the allowance against your actual incident history before committing.
How the calculation works
Downtime allowance is simply the unavailability fraction multiplied by the period. 99.9% permits 0.1% of a year, which is 8 hours 45.6 minutes, while 99.99% permits 52.6 minutes and 99.999% just 5.26 minutes. Each additional nine divides the allowance by ten while multiplying the redundancy and staffing required to achieve it.
Contracts differ on what counts. Scheduled maintenance is often excluded, availability may be measured monthly rather than annually, and partial degradation may not count as downtime at all. Composite systems also compound: three dependencies at 99.9% each, all required, give roughly 99.7% combined — which is why a service can only credibly promise less availability than the weakest link it depends on.
Allowed downtime = (1 − uptime%) × period ; serial dependencies: A_total = A₁ × A₂ × A₃Source: Standard SLA availability arithmetic; serial availability multiplication per reliability engineering practice.
Worked example
A team is asked to commit to 99.95% monthly availability while depending on two providers at 99.99% and 99.9%.
- Monthly allowance at 99.95% = 0.0005 × 43,200 minutes = 21.6 minutes.
- Dependency ceiling = 0.9999 × 0.999 = 0.99890, i.e. 99.89%.
- That ceiling alone allows 47.5 minutes of downtime a month.
21.6 minutes a month is the target, but the dependency chain already permits 47.5 — the commitment is not achievable without redundancy on the weaker provider.
Frequently asked questions
What uptime is realistic for a small service?+
99.9% is achievable for a single well-run service with automated deploys and monitoring, allowing about 44 minutes of downtime a month. Higher targets need multi-zone redundancy and staffed on-call.
Does scheduled maintenance count against the SLA?+
Often not, if the agreement defines announced maintenance windows as excluded. Read the definition carefully — it is the difference between a comfortable and an impossible commitment.
Why does my availability drop when I add a dependency?+
Serial dependencies multiply. Two components each at 99.9% give 99.8% combined, so every required third-party service lowers your achievable ceiling unless you can fail over.
Is annual or monthly measurement better?+
Monthly is stricter, since one long outage cannot be diluted across a year. Providers usually prefer monthly windows with service credits; customers should confirm which period applies.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.