Probability
System Availability Probability Calculator
Calculate single-unit inherent availability, parallel-system availability, operational availability after planned downtime, annual unavailable hours, downtime cost, and margin to target.
Single-unit inherent availability-
Single-unit unavailability fraction-
Parallel-system unavailability fraction-
Parallel-system inherent availability-
Planned downtime fraction-
Operational availability after planned downtime-
Expected unavailable hours per year-
Expected annual downtime cost-
Margin to availability target-
Decision view
Annual uptime budget and redundancy response
Annual uptime budget and redundancy responseThe service-year strip allocates operating and unavailable hours, while a redundancy curve shows the theoretical availability gain from each independent parallel unit and compares the result with the entered target.
Exact scenario comparisonMean time to repair, MTTR (hours) changes while all other entered assumptions remain constant.
| Mean time to repair, MTTR (hours) | Single-unit inherent availability | Single-unit unavailability fraction | Parallel-system unavailability fraction | Parallel-system inherent availability | Planned downtime fraction | Operational availability after planned downtime | Expected unavailable hours per year | Expected annual downtime cost | Margin to availability target |
|---|
How to use System Availability Probability Calculator
- Enter MTBF and MTTR on the same hour basis.
- Enter the number of independent active parallel units.
- Add the annual service window, planned downtime, cost, and target.
Calculator guide
Understanding System Availability Probability Calculator
Availability depends on how long a repairable unit runs and how quickly it returns to service. Independent parallel units can reduce random unavailability, while planned downtime remains a separate annual deduction.
Detailed calculation process
Detailed repairable-system availability calculation
The default case uses 1,200-hour MTBF, 4-hour MTTR, two independent parallel units, and 12 planned hours per 8,760-hour service year.
General formula: A_1=MTBF/(MTBF+MTTR)q_1=1-A_1A_n=1-q_1^nA_o=A_n(1-D_p/H)U=H(1-A_o) Random unavailability is reduced by independent parallel capacity; planned downtime then reduces the remaining operating fraction.
What each symbol means
Worked substitution with the default inputs
Operational uptime plus expected unavailable hours reconciles to the entered 8,760-hour service year.
Worked situations
Practical examples
- MTBF of 1,200 hours and MTTR of 4 hours gives 99.6678% inherent availability for one unit.
- Two independent units in parallel reduce random unavailability to roughly 0.001104%, before planned downtime.
Better inputs
Useful tips
- Use observed failure and restoration data from the same boundary.
- Separate planned maintenance from random repair time.
- Apply redundancy only when units can carry service independently.
Before relying on the result
Limitations and common mistakes
- Parallel-unit failures are assumed independent.
- Common power, network, software, staffing, and environmental failures are not modeled.
- MTBF and MTTR averages hide repair-time and failure-rate distributions.
Reference
Key terms
- MTBF
- Mean operating time between repairable failures.
- MTTR
- Mean time required to restore service.
- Inherent availability
- Availability from failure and repair behavior before planned downtime.
- Common-cause failure
- One cause that can defeat multiple redundant units together.
Important note
Do not claim redundancy benefit without analyzing shared dependencies and failover behavior.
Frequently asked questions
Does a second unit always deliver squared unavailability?
Only when the units are independent and either can carry the required service.
Why separate planned downtime?
Maintenance windows are not random repair events and may affect every redundant unit.
Is availability the same as reliability?
No. Reliability is failure-free operation over time; availability also credits repair and restoration.