How to use
- Pick a mode — SLA → downtime, Downtime → SLA or Combined availability.
- Enter an availability percentage (or click a common SLA), or enter a downtime and period.
- For combined availability, give each component its availability and number of redundant copies to see the whole system's availability and allowed downtime.
Options explained
- Period
- A month is 365.25 ÷ 12 days and a year is 365.25 days, as most SLA calculators use.
- Nines
- 99.9% is three nines; 99.95% is about 3.3 nines.
- Redundant copies
- Copies of one component where any one is enough, like a primary and standby database. System availability is the product of components; each component is 1 − (1 − single-copy availability)^copies.
Allowed downtime by SLA
Months use 365.25 ÷ 12 days and years 365.25 days, so these figures match the calculator:
| Availability | Per day | Per month | Per year |
|---|---|---|---|
| 99% | 14 min 24 s | 7 h 18 min 18 s | 3 d 15 h 39 min 36 s |
| 99.9% | 1 min 26.4 s | 43 min 49.8 s | 8 h 45 min 57.6 s |
| 99.95% | 43.2 s | 21 min 54.9 s | 4 h 22 min 58.8 s |
| 99.99% | 8.6 s | 4 min 23 s | 52 min 35.8 s |
| 99.999% | 0.9 s | 26.3 s | 5 min 15.6 s |
A worked example
In Downtime → SLA, 30 minutes of downtime in a month is 1 − 30 ÷ 43,830 minutes = 99.93% availability, or about 3.2 nines. In Combined availability, two redundant 99.9% web servers give 1 − 0.001² = 99.9999%. Put them in series with one 99.95% database and multiply: 99.9999% × 99.95% ≈ 99.95%. The single database caps the whole system.
Frequently asked questions
What is the difference between 99.9% and 99.99%?
99.9% allows about 8 h 46 min of downtime a year, 99.99% about 52 minutes and 99.999% only about 5 minutes.
Why does adding components in series lower availability?
In series, any single failure takes the system down, so availabilities multiply. Two 99.9% components in series give about 99.8%.
How accurate is the redundancy math?
It assumes copies fail independently. Shared power, a shared bad config or the same bug can take copies down together, so real availability is usually lower.
Privacy
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