Availability (Uptime)
Percentage of time a system is operational.
Formula
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Example
MTBF 1000 h, MTTR 1 h → A = 99.9%
How it works
Availability tells you what fraction of time a system is expected to be running rather than down. It comes from comparing the average operating time between failures (MTBF) to the total cycle of operating time plus repair time, so the formula measures "good time" divided by "good time + bad time." Multiplying by 100 converts that fraction into a percentage.
When to use it
Use this when you need to estimate how reliable or accessible equipment, servers, networks, or industrial machines are over time. It is especially useful for maintenance planning, service-level targets, and comparing systems with different failure and repair patterns.
Worked example, step by step
Suppose a server has MTBF = 500 hours and MTTR = 5 hours. A = MTBF / (MTBF + MTTR) × 100 A = 500 / (500 + 5) × 100 A = 500 / 505 × 100 A ≈ 0.9901 × 100 A ≈ 99.01% So the server is expected to be available about 99.01% of the time.
Common mistakes to avoid
- Using MTTR as if it were total downtime over a year instead of the average repair time per failure
- Forgetting to add MTBF and MTTR in the denominator before converting to a percentage
- Mixing units for MTBF and MTTR, such as hours for one and days for the other
Did You Know?
"Five nines" (99.999%) availability allows only about 5 minutes of downtime a year.
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