Uptime / SLA Calculator
Turn an SLA percentage into the downtime it allows per day, month, and year - or work backwards from real downtime. Includes error budgets and composite availability for dependent services.
99.9% is "three nines". This allowed downtime is your error budget for each period - spend it on incidents, deploys, and maintenance.
Target uptime: 99.9% (three nines)
Allowed downtime:
Per day: 1m 26.4s
Per week: 10m 4.8s
Per month: 43m 12.0s
Per year: 8h 45m 36sWhat uptime, SLA, and "the nines" mean
Uptime is the share of time a service is available. An SLA (Service Level Agreement) promises a minimum, usually as a percentage like 99.9%. People count the leading nines: 99.9% is "three nines", 99.999% is "five nines". Each extra nine cuts the allowed downtime by roughly 10x.
How allowed downtime is calculated
It is simply the period length times the fraction you are not promising: downtime = period × (1 − SLA/100). For 99.9% over a 30-day month: 2,592,000 s × 0.001 = 2,592 s = 43m 12s. The window matters - a "monthly" SLA and an "annual" SLA allow very different absolute downtime, which is why this tool lets you pick the month and year length.
Error budgets
SRE teams flip the SLA around: the allowed downtime is your error budget. If your target is 99.9%, you have ~43 minutes a month to "spend" on incidents, risky deploys, and maintenance. When the budget is healthy you ship faster; when it is nearly gone you slow down and stabilize. The burn rate is how fast an ongoing incident is consuming it.
Composite availability: dependencies multiply
When a request depends on several services in series, it is only up when all of them are - so availabilities multiply. Three components at 99.9% give 0.999 × 0.999 × 0.999 ≈ 99.7%, not 99.9%. More dependencies means a lower ceiling, which is why critical paths are kept short and redundant.
SLA vs SLO vs SLI
- SLI - the measured indicator (e.g. % of successful requests).
- SLO - your internal target for that indicator (e.g. 99.95%).
- SLA - the external promise, usually looser than the SLO, often with penalties if missed.
Gotchas
- Measurement window. Monthly vs annual SLAs allow very different downtime; always check which your provider uses.
- Scheduled maintenance. Many SLAs exclude planned maintenance windows from the downtime count.
- What counts as "down". Partial outages, elevated errors, and slow responses may or may not count - the SLI definition decides.
The "nines" - allowed downtime by SLA
| SLA | Nines | Per year | Per month | Per day |
|---|---|---|---|---|
| 99% | Two | 3d 15h 36m | 7h 12m | 14m 24s |
| 99.9% | Three | 8h 45m 36s | 43m 12s | 1m 26.4s |
| 99.95% | Three and a half | 4h 22m 48s | 21m 36s | 43.2s |
| 99.99% | Four | 52m 33.6s | 4m 19.2s | 8.6s |
| 99.999% | Five | 5m 15.4s | 25.9s | 0.86s |
Frequently asked questions
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