MTBF calculator
Series-system failure rate from up to eight lines of quantity and rate, converted to MTBF, FIT, annualized failure rate and the probability of surviving a mission.
- MTBF, hours
- 4,167,000 h
- Total failure rate
- 240 FIT
- Total failure rate, per 10⁶ hours
- 0.24
- Probability of no failure over 43800 h
- 98.95 %
- Probability of at least one failure
- 1.046 %
- Annualized failure rate (8760 h)
- 0.21 %
| Line | Quantity | Rate each | Line FIT | Share |
|---|---|---|---|---|
| Line 1 | 1 | 100 FIT | 100 | 41.7 % |
| Line 2 | 4 | 25 FIT | 100 | 41.7 % |
| Line 3 | 20 | 2 FIT | 40 | 16.7 % |
How this is calculated
λ_total = Σ N_i · λ_i MTBF = 1 / λ_total = 10⁹ / FIT hours R(t) = exp( −λ_total · t ) AFR = 1 − exp( −λ_total · 8760 h ) 1 FIT = 1 failure per 10⁹ h = 0.001 per 10⁶ h
- Assumes
- A constant failure rate (the flat part of the bathtub curve) and a series system: any one part failing fails the product. Infant mortality and wear-out are not modeled.
- Rates
- You supply them. No handbook rates are built in. Vendor reliability reports give FIT at a stated temperature and confidence level; MIL-HDBK-217F gives generic rates per 10⁶ h with quality factors.
- AFR
- Probability of a failure within 8760 powered hours. For a product powered part of the time, scale the mission time, not the rate.
- MIL-HDBK-217F, Reliability Prediction of Electronic EquipmentNotice 2, 28 February 1995, Appendix A, Equation 1
- NIST/SEMATECH e-Handbook of Statistical MethodsSection 8.1.6.1, Exponential
- TI SNAA391, High Reliable BAW Oscillator MTBF and FIT Rate CalculationsJune 2023
Questions
MTBF in hours is 10⁹ divided by FIT. TI SNAA391 works 1.5 FIT to 6.7 × 10⁸ hours. FIT per 10⁶ hours is FIT divided by 1000.
No. It means a failure rate of 1000 FIT. Over one year of continuous use, about 0.87 % of units fail; over 5 years, about 4.3 %. It says nothing about wear-out life.
MIL-HDBK-217F Appendix A sums quantity times failure rate over each part type for an early estimate, before stresses are known. The part stress method in its main sections refines it with temperature, voltage and environment.
exp(−λt). At 240 FIT, the default here, the chance of no failure over 43,800 powered hours (5 years) is about 99.0 %.