# Calculating service availability

# with MTBF, MTTD, MTTR and MTTF

**MTBF**, **MTTD**, **MTTR** and **MTTF** are the four parameters required to calculate the availability of a service or an individual component in a specific architecture.

- MTBF is the
**Mean Time Between Faults** - MTTD is the
**Mean Time To Detection** - MTTR is the
**Mean Time To Repair** - MTTF is the
**Mean Time To Failure**

Well, once we got this, there are just a few little formula to know :

MTBF = MTTD + MTTR + MTTF

Availabiliy = MTBF / (MTBF + MTTR)

For example, a component with a MTBF of 8 days and a MTTR of 3 hours will have:

**Availability** = (8*24) / (8*24 + 3) = **0.985**

A good way to understand availability is to look at the downtime per year they represent:

- 90% availability = 36 days and 12 hours downtime/year
- 99% availability = 87 hours, 36 minutes downtime/year
- 99.9% availability = 8 hours,45 minutes, 36 seconds downtime/year
- 99.99% availability = 52 minutes,33 seconds downtime/year
- 99.999% availability = 5 minutes,15 seconds downtime / year
- 99.9999% availability = 31 seconds downtime/year

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