Free reliability calculator
Uptime SLA & Node Redundancy Calculator
A time-based uptime target limits unavailability to period length multiplied by one minus the target percentage. For example, 99.9% availability over 30 days allows 43 minutes 12 seconds of downtime. This calculator also shows a mathematical active-active redundancy estimate when node failures are assumed to be independent.
Last updated: July 19, 2026 · No sign-up · Runs in your browser
Availability assumptions
Downtime budget
Allowed downtime at target
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Independent-node estimate
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Estimated downtime in period
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Independent nodes needed to meet target mathematically
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Availability formula and common targets
Allowed downtime is period seconds × (1 − target availability). For independent active-active nodes where any one node can serve the workload, estimated service availability is 1 − (1 − per-node availability)^node count. Real systems commonly share dependencies and failure modes, so the redundancy result is labeled as a mathematical scenario, not an observed SLA.
| Availability | Allowed per 30 days | Allowed per 365 days |
|---|---|---|
| 99% | 7h 12m | 3d 15h 36m |
| 99.9% | 43m 12s | 8h 45m 36s |
| 99.95% | 21m 36s | 4h 22m 48s |
| 99.99% | 4m 19s | 52m 34s |
| 99.999% | 26s | 5m 15s |
Primary reliability reference
Google SRE: Availability Table converts time-based availability levels into allowed unavailability windows and notes that aggregate operation-based availability can be more useful for partially available or variable-load services.
Frequently asked questions
How much downtime does 99.9% uptime allow per month?
For a 30-day month, 99.9% time-based availability allows 43 minutes 12 seconds of unavailability. Calendar months vary in length, so use the exact reporting period defined by the SLO or contract.
How much downtime does 99.99% uptime allow per year?
Over 365 days, 99.99% availability allows about 52 minutes 34 seconds of downtime. Whether planned maintenance counts depends on the specific SLI, SLO, or SLA definition.
Does adding a second node automatically improve uptime?
No. Redundancy helps only when another healthy node can serve traffic and failures are not dominated by shared dependencies, configuration, software, network, or operational mistakes.
What is the difference between an SLI, SLO, and SLA?
An SLI is the measured reliability indicator, an SLO is the target for that indicator, and an SLA is a formal agreement that can include remedies. Exact definitions and measurement windows matter more than the number of nines alone.
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