Reliability · Availability

Composite SLA calculator.

List the availability of each component and see the combined SLA for hard dependencies in series, or for independent redundant copies in parallel.

AWS Well-Architected methodSeries and parallelRuns in your browserChecked 15 October 2026

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How to calculate a composite SLA

AWS's Well-Architected Reliability Pillar gives both rules. Where a system has hard dependencies, its availability is the product of the availabilities: a 99.99% system with two independent 99.99% hard dependencies reaches 99.99% x 99.99% x 99.99% = 99.97%. For independent redundant components, availability is 100% minus the product of the failure rates: two 99.9% components give 100% - (0.1% x 0.1%) = 99.9999%.

These are theoretical ceilings. They assume failures are independent and failover is instant and perfect. Shared causes (a common region, a bad deploy, a shared database) make real availability lower, and a cloud vendor SLA is a financial-credit promise, not a guarantee. Series mode with replicas applies the parallel rule to each stage first.

Frequently asked questions

How do I calculate the SLA of dependent services?

Multiply their availabilities. Three 99.9% services in a chain give about 99.7%.

How does redundancy improve availability?

For independent copies, multiply the unavailabilities. Two 99.9% copies give 99.9999%.

Is this accurate for real systems?

It is an upper bound. Correlated failures, failover time and shared dependencies reduce it.

What about soft dependencies?

A soft dependency, where failure is compensated for in the application, should be left out of the chain (AWS definition).

Sources

Checked15 October 2026

Formulas follow the sources above. Periods use a 30-day month and a 365-day year. Runs entirely in your browser; nothing you type is sent or stored.

Scope

Planning estimates only. Your SLA contract defines its own measurement window, exclusions and remedies, and those override these generic figures. Theoretical availability assumes independent failures.