Composite Service Availability Calculator

Work out composite service availability instantly with clear inputs, formula shown and shareable results.

About Composite Service Availability Calculator

Components in series multiply their availabilities, so five dependencies at 99.9 percent give only 99.5 percent overall — worse than any individual part. Redundancy works the other way: two parallel replicas fail only when both fail, giving 1 - (1-a)^2. Combining the two is how a chain of ordinary components delivers an extraordinary service level.

Formula

Composite availability

series: A = product of component availabilities; parallel: A = 1 - (1 - a)^replicas; combined = (parallel availability)^series count

Worked example

With the default values below, Composite Service Availability Calculator returns Composite availability of 99.99950%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Availability of each componentaccepts 50–100 %: 99.9 %
Components in series (all must work)accepts 1–100: 5
Redundant replicas per componentaccepts 1–20: 2

Results

Composite availability: 99.99950%
Availability with no redundancy: 99.50100%
Availability of one redundant component: 99.99990%
Annual downtime: 2.6 min

Step by step

1Values used
2Composite availability
3Composite availability= 99.99950
4Availability with no redundancy= 99.50100
5Availability of one redundant component= 99.99990
6Annual downtime= 2.6 min

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is my service less available than every dependency?

Because failures compound in series. Each additional required dependency subtracts from the total, which is the mathematical argument for graceful degradation.

Does redundancy always multiply availability?

Only if failures are independent. Shared power, shared control planes and correlated software bugs defeat replica maths entirely, which is why availability zones exist.