MTTR / MTBF / Availability Calculator

MTTR (Mean Time To Recovery/Repair) measures the average time it takes to restore service after a failure — a lower MTTR means faster recovery. MTBF (Mean Time Between Failures) measures the average time between two consecutive failures — a higher MTBF means more reliable service. Together they determine availability: Availability = MTBF / (MTBF + MTTR).

99% (two nines) = ~87.6 hours downtime/year. 99.9% (three nines) = ~8.76 hours/year. 99.95% = ~4.38 hours/year. 99.99% (four nines) = ~52 minutes/year. 99.999% (five nines) = ~5.26 minutes/year. Most enterprise SLAs target 99.9%–99.99%, while consumer apps often target 99.9%.

Measure MTTR from the moment the incident is detected (alert fires) to the moment service is fully restored (not just temporarily fixed). Track: detection time, acknowledgement time, mitigation time, and full resolution time separately. Many teams confuse MTTD (detection), MTTA (acknowledgement), and MTTR, leading to misleading metrics.

For high-traffic web services, targeting MTBF of 720+ hours (30 days) is common for P1+ incidents. MTBF below 168 hours (1 week) indicates systemic reliability issues requiring architectural review. MTBF should be measured separately per service and per severity level — mixing them obscures real problem areas.

Key MTTR improvement levers: (1) Better observability — faster detection and diagnosis with distributed tracing and structured logs. (2) Runbooks — documented step-by-step recovery procedures. (3) Automated rollbacks — deploy systems that can auto-revert on health check failure. (4) Game days — practice incident response with chaos engineering. (5) Post-mortems — systematic review of each incident to remove recurring causes.