Network Availability
Calculate network uptime percentage and allowed downtime from SLA availability targets.
Inputs
Comma-separated availability percentages for each component, e.g. 99.9, 99.95, 99.99
Overall Availability
99.84006%
Classification
Two nines (99%+)
Allowed Annual Downtime
14.02 hours
Allowed Annual Downtime
841.19minutes
Step by step
Values used
Topology = Series (all components must work — no redundancy); Component Availabilities (%) = 99.9, 99.95, 99.99
Series availability
A_series = A1 × A2 × A3 × ...
Parallel (redundant) availability
A_parallel = 1 − (1−A1) × (1−A2) × ...
Overall Availability
= 99.84006
Classification
= Two nines (99%+)
Allowed Annual Downtime
= 14.02 hours
Allowed Annual Downtime
= 841.19 minutes
How it works
Combining component availabilities depends entirely on topology. In series (no redundancy — every component must be up for the whole system to be up), overall availability is the product of each component's availability, so reliability always degrades as more components are chained without redundancy. In parallel (redundant paths — the system is up if at least one component is up), overall availability is 1 minus the product of each component's failure probability, so redundancy dramatically improves availability even with individually less-reliable components.
Formulas
Series availability
A_series = A1 × A2 × A3 × ...
- A_i
- Availability of component i, as a fraction
Parallel (redundant) availability
A_parallel = 1 − (1−A1) × (1−A2) × ...
Frequently Asked Questions
Why does adding more components in series reduce availability?
Every additional link in a series chain adds another point of failure that can take down the entire path, and since each component's availability fraction is less than 1, multiplying more of them together always produces a smaller overall product.
Why does redundancy (parallel) help so much?
With parallel paths, the whole system only fails if every redundant path fails simultaneously — an event whose probability is the product of each path's (typically small) failure probability, making total failure exponentially less likely as you add more independent paths.
What do 'three nines' or 'five nines' mean?
They refer to the count of consecutive 9s in the availability percentage: 99.9% ('three nines') allows about 8.76 hours of downtime per year, while 99.999% ('five nines') allows only about 5.26 minutes per year — each additional nine is roughly a 10x improvement.
Does this model assume independent failures?
Yes — both formulas assume each component fails independently of the others. Correlated failures (e.g. a single power outage taking down two 'redundant' devices in the same rack) violate this assumption and can make real-world availability worse than the calculated parallel figure suggests.