Network Availability
Calculate network uptime percentage and allowed downtime from SLA availability targets.
About Network Availability
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) × ...
Worked example
With the default values below, Network Availability returns Overall Availability of 99.84006%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Topologyone of Series (all components must work — no redundancy), Parallel (redundant — any one component working is enough)
- Series (all components must work — no redundancy)
- Component Availabilities (%)
- 99.9, 99.95, 99.99
Results
- Overall Availability
- 99.84006%
- Classification
- Two nines (99%+)
- Allowed Annual Downtime
- 14.02 hours
- Allowed Annual Downtime
- 841.19 minutes
Step by step
- 1Values used
- 2Series availability
- 3Parallel (redundant) availability
- 4Overall Availability= 99.84006
- 5Classification= Two nines (99%+)
- 6Allowed Annual Downtime= 14.02 hours
- 7Allowed Annual Downtime= 841.19 minutes
Assumptions and limits
- The result is derived only from the 2 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 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.