Network Availability Calculator
Convert availability percentages into real downtime, calculate the availability of series and parallel components, and see what redundancy actually buys once shared failure modes are included.
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Components in series multiply
A request passing through a load balancer, a firewall, an application server and a database depends on all four, so the availabilities multiply. Four components at 99.9 percent each give 99.6 percent overall, which is 35 hours of downtime a year rather than nine. Every component added to the path lowers the total, which is why simplifying an architecture improves availability even when each individual part is unchanged.
Redundancy helps far less than the arithmetic suggests
Two components in parallel at 99.9 percent each give 99.9999 percent in theory, which is thirty seconds of downtime a year. Real redundant pairs do not achieve anything like that, because the failure modes are correlated: both are in the same rack, on the same power feed, running the same software version with the same bug, configured by the same person from the same template. The common cause failure rate is what actually determines the outcome, and it is usually far larger than the independent failure rate.
The nines are not evenly spaced
Each additional nine costs roughly ten times more than the last while removing ninety percent of the remaining downtime. Moving from 99 to 99.9 percent removes about 79 hours a year. Moving from 99.99 to 99.999 removes about 47 minutes. That final step usually requires geographic redundancy, automated failover with no human in the loop, and a deployment process that cannot cause an outage, which is a different class of engineering rather than more of the same.
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Frequently Asked Questions
How much downtime does each nine allow?
99 percent allows 3.65 days a year, 99.9 allows 8.77 hours, 99.99 allows 52.6 minutes, 99.999 allows 5.26 minutes and 99.9999 allows 31.6 seconds. Each nine removes 90 percent of the remaining downtime.
Why does adding components lower availability?
Because components in series all have to work, so their availabilities multiply. Four independent components at 99.9 percent give 99.6 percent together, which is four times the downtime of any one of them.
Does redundancy really give six nines?
Only if the failures are independent, which they rarely are. Two servers in the same rack share power, cooling, network and a software version. The common cause failure rate dominates, and the calculator lets you enter it because ignoring it produces a number that is meaningless.
Is planned maintenance counted?
That depends on the agreement. Most commercial SLAs exclude announced maintenance windows, which is why a service can report 99.99 percent while being unavailable for four hours a month. Read the exclusions before comparing numbers.
What availability should I target?
Whatever the business consequence of downtime justifies. Each nine costs roughly ten times the last, and beyond 99.99 percent the constraint is usually the deployment process and human response time rather than hardware. Many services do better by reducing the impact of failure than by trying to prevent it.
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How to Use
Enter component availabilities and how they are arranged to get the system availability.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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