System Availability Calculator
Calculate the availability of a device or system from its mean time between failures and mean time to repair, and see the expected downtime per year.
- A = MTBF ÷ (MTBF + MTTR) = 50,000 ÷ 50,008 = 99.9840 %
- Redundant pair (independent): A = 1 − (1 − A)²
Availability Formulas
Two in parallel: A = 1 − (1 − A)²
Two in series: A = A1 × A2
Worked example
A controller with 50,000 h MTBF and 8 h MTTR is 99.984 % available, about 1.4 hours down per year. A redundant pair reaches 99.99999 %.
Frequently Asked Questions
What is MTTR?
Mean time to repair: the average time to detect, diagnose, repair and restore a failed item.
What does five nines mean?
99.999 % availability, about 5.3 minutes of downtime per year.
Why does redundancy improve availability so much?
Both units must fail at the same time for the system to be down, which is far less likely, provided failures are independent and repairs are quick.
Learn more on the blog
PLC redundancy and hot standby
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article