Turn 99.9% or 99.99% into allowed downtime per month and year, and combine availability.
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The SLA Uptime Calculator converts percentages such as 99.9% and 99.99% into allowed downtime per day, week, month and year, and works backwards from real downtime to availability. Formula: allowed downtime = period × (1 − availability). It also offers a "nines" reference table, combined availability for series and parallel designs, error budget and burn rate, maintenance exclusion, and tiered SLA service credits (with an Amazon S3 Standard template and custom tiers).
Allowed downtime = period × (1 − availability). Over a 30-day month, 99.9% is about 43.2 minutes and 99.99% about 4.32 minutes; over a 365-day year, 99.9% is about 8.76 hours. The "nines" table lists the figures per day, week, month and year.
Because "a month" is defined differently: 30 days, 31 days or an average of 30.44 days give different results. This tool defaults to a 30-day month and a 365-day year and lets you switch to the average month and a 365.25-day year; cloud providers also define their own measurement windows, so follow the contract.
Series (every tier must be up): total = product of the tiers' availability. Parallel (one healthy replica is enough): 1 − (1 − single availability)^replicas. Two 99% servers in parallel reach 99.99%, but with a 99.9% database chained in series the whole cannot exceed 99.9%. The tool also points out the weakest tier.
An error budget is how much failure the target SLO allows over a period: period × (1 − SLO). Burn rate = (consumed ÷ budget) ÷ (elapsed ÷ period): 1 means you use it up exactly on schedule, above 1 means it runs out early. The tool projects on which day the budget would be exhausted at the current pace.
Many SLAs exclude announced, scheduled maintenance from the calculation. The tool's "including maintenance" and "excluding maintenance" figures differ by removing the maintenance time from both the downtime and the measured window; whether your contract allows exclusion, and how much, depends on its terms.
Enter the month's measured availability and bill; the tool applies the tiers (below a threshold, the matching percentage is credited) to give the amount. The built-in template is the public Amazon S3 Standard SLA (the page date and our verification date are shown below), and you can switch to custom tiers. Actual eligibility, claim deadlines and exclusions follow the provider's contract.
The series/parallel formulas assume independent failures and instant failover. In reality shared power, same-zone or same-vendor outages, software bugs, misconfiguration and human error often hit several components together, so real availability is usually below the theoretical value. The tool is for planning and comparison, not measurement.
| Uptime | Per day | Per week | Per month (30 d) | Per year (365 d) |
|---|---|---|---|---|
| 90% | 2 h 24 min | 16 h 48 min | 3 d | 36 d 12 h |
| 95% | 1 h 12 min | 8 h 24 min | 1 d 12 h | 18 d 6 h |
| 99% | 14 min 24 s | 1 h 40 min 48 s | 7 h 12 min | 3 d 15 h 36 min |
| 99.5% | 7 min 12 s | 50 min 24 s | 3 h 36 min | 1 d 19 h 48 min |
| 99.9% | 1 min 26.4 s | 10 min 4.8 s | 43 min 12 s | 8 h 45 min 36 s |
| 99.95% | 43.2 s | 5 min 2.4 s | 21 min 36 s | 4 h 22 min 48 s |
| 99.99% | 8.64 s | 1 min 0.48 s | 4 min 19.2 s | 52 min 33.6 s |
| 99.995% | 4.32 s | 30.24 s | 2 min 9.6 s | 26 min 16.8 s |
| 99.999% | 0.864 s | 6.048 s | 25.92 s | 5 min 15.36 s |
| 99.9999% | 0.086 s | 0.605 s | 2.592 s | 31.536 s |
