MTTF Calculator

Mean Time To Failure (MTTF) is used for non-repairable items that are replaced rather than repaired when they fail (e.g., capacitors, LED bulbs, hard drives). MTBF (Mean Time Between Failures) applies to repairable systems. For non-repairable items, the cycle is: operate until failure, then replace, not repair. MTTF = total_operating_hours / number_of_failures.

Place N items on test, run until all have failed or until a predetermined time (truncated test). MTTF = total_unit_hours / total_failures. Total unit hours = sum of all individual operating times until failure. For a truncated test where some units have not yet failed, those units contribute their censored (right-censored) operating time, but statistical methods such as maximum likelihood estimation are needed for precision.

L10 life (also B10 life) is the time by which 10% of a population will have failed - equivalently, the time at which 90% survival is expected. For exponential distributions: L10 = -ln(0.90) / lambda = 0.1054 / lambda. L10 = 0.1054 * MTTF. L10 is commonly used for bearings, drives, and electrolytic capacitors. L50 (median life) = 0.693 * MTTF for exponential distributions.

No. For series components, the system failure rate = sum of individual failure rates, not the MTTF values. Equivalently, 1/MTTF_system = 1/MTTF_1 + 1/MTTF_2 + ... For example, two components each with MTTF of 10,000 hours in series have a system MTTF of 5,000 hours, not 20,000 hours. This is a common mistake in reliability calculations.

The Arrhenius model predicts that MTTF decreases exponentially with temperature: MTTF(T) = MTTF(T0) * exp(Ea/k * (1/T - 1/T0)), where Ea is the activation energy (typically 0.5-1.0 eV for electronics), k is Boltzmann's constant, and T is the absolute temperature. A 10C increase in junction temperature roughly halves the lifetime of many electronic components.