Engineering
MTBF, MTTR, System Availability & SLA "Nines" downtime budget formulas
Calculate observed availability and a time-based downtime budget for the supplied window. Distinguish sample repair statistics from future reliability and contractual SLA rules.
Translate SRE outage logs into MTBF, MTTR, and SLA "Nines" error budgets!
For a stated observation window, observed availability is (window hours-downtime)/window hours. With positive failure count, uptime/failures and downtime/failures give sample mean uptime and mean downtime; their ratio recovers the same observed availability. A zero failure count does not prove future perfect reliability, and availability can still be below one if downtime occurred without a counted failure.
This worksheet compares observed downtime with a chosen time-based target for the same window. The 8760-hour example represents a non-leap 365-day year. Real SLAs may use monthly windows, request-based measures, exclusions and claim rules; this worksheet does not establish contractual breach or credit entitlement.
| Reliability Metric | Formula / Input | Value | Operational Meaning |
|---|---|---|---|
| Observation Window (B2) | 8,760 hrs (1 yr) | 4 outages (B3), 6 hrs down (B4) | 8,754 hrs healthy uptime |
| MTBF & MTTR | (B2-B4)/B3 & B4/B3 | 2,188.5 hrs | 1.5 hrs | 90 min mean recovery |
| Availability A | MTBF / (MTBF + MTTR) | 0.999315 (99.9315%) | Exceeds 99.9%, misses 99.99%! |
| 99.99% SLA Budget (B5) | (1 - 0.9999) * 525600 | 52.56 min/yr allowed | Actual 360 min exceeds the illustrative target budget |
Observed availability for the supplied window
Compute availability directly from positive window hours and downtime in [0,window]. Failure count may be zero; MTBF and MTTR cannot be estimated by division when it is zero.
=IF(COUNT(B2:B5)<>4,"Invalid inputs",IF(OR(B2<=0,B3<0,B3<>INT(B3),B4<0,B4>B2,B5<0,B5>1),"Invalid inputs",ROUND((B2-B4)/B2,6)))Allowed downtime minutes for this window
Multiply (1-target) by B2*60. This keeps the comparison on the actual supplied observation period, including a leap-year or monthly period if entered. Compare unrounded values; round only for display.
=IF(COUNT(B2:B5)<>4,"Invalid inputs",IF(OR(B2<=0,B3<0,B3<>INT(B3),B4<0,B4>B2,B5<0,B5>1),"Invalid inputs",ROUND((1 - B5) * (B2*60), 2)))Try the example
Use the original values or change them to test your understanding.