Availability & SLA
Model availability from MTBF/MTTR or per-component percentage and its redundancy (series, parallel or N+1). Get the nines and the downtime per year, month and week.
Per-component contribution
| Component | A (%) | Nines | Downtime / year |
|---|
The nines of availability
| SLA | Nines | Downtime / year | Downtime / month | Downtime / week |
|---|
Periods: year = 525,600 min (365 d), month = 43,800 min (year/12), week = 10,080 min (7 d). MTBF/MTTR must be in the same time units. The model assumes independent, uncorrelated failures.
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How it works
The calculator models the availability of a system from its components. Each component is entered either via MTBF/MTTR (A = MTBF / (MTBF + MTTR), in the same time units) or directly as a percentage. It then combines them by topology: in series everything must work (A = ∏ Aᵢ, availability drops), in parallel one unit is enough (A = 1 − ∏(1 − Aᵢ), availability rises) and for k-of-n with identical units it uses the binomial sum A = Σ C(n,i)·aⁱ·(1−a)ⁿ⁻ⁱ for i from k to n, which covers N+1 schemes.
With the availability solved it reports the nines (−log₁₀(1 − A)) and the expected downtime per year (525,600 min), month (43,800 min) and week (10,080 min): downtime = (1 − A) × period. The model assumes independent, uncorrelated failures, the standard assumption in reliability math; a common-cause failure (power, fire) can take down redundant branches at once.
Example: server + switch + WAN link in series
- Per-component availability: server
50000/(50000+4) ≈ 99.9920 %, core switch200000/200002 ≈ 99.9990 %, WAN link20000/20006 ≈ 99.9700 %. - In series they multiply:
A ≈ 99.961 %, about 3.4 nines. - Annual downtime:
(1 − 0.99961) × 525600 ≈ 205 min, roughly 3 h 25 min per year, dominated by the WAN link.