MTTF Calculator - Mean Time To Failure
Free professional calculator for non-repairable system reliability analysis with MTTF vs MTBF comparison
MTTF Calculator
Complete Guide to MTTF (Mean Time To Failure)
What is MTTF?
Mean Time To Failure (MTTF) measures the average time a non-repairable system operates before it fails permanently. Unlike MTBF, MTTF applies to components that are replaced rather than repaired — such as light bulbs, bearings, or semiconductor chips.
- Applies to non-repairable items only
- Predicts component lifespan
- Drives replacement scheduling
- Used in warranty period calculations
- Essential for spare parts inventory
MTTF Formula
Basic Formula:
MTTF = Total Operating Time ÷ Number of UnitsIndividual Data:
MTTF = (t₁ + t₂ + ... + tₙ) ÷ nWhere each tᵢ is the time-to-failure for unit i, and n is the total number of units that failed.
MTTF vs MTBF — Key Differences
| Feature | MTTF | MTBF |
|---|---|---|
| System Type | Non-repairable | Repairable |
| After Failure | Replaced entirely | Repaired and returned to service |
| Examples | Light bulbs, bearings, fuses | Machines, vehicles, servers |
| Formula | Total time ÷ units failed | Total time ÷ number of failures |
| Use Case | Component replacement planning | Maintenance scheduling |
MTTF Calculation Example
LED Light Bulb Testing:
A manufacturer tests 5 LED bulbs until failure and records their lifetimes:
Individual Lifetimes:
- • Bulb 1: 25,000 hours
- • Bulb 2: 30,000 hours
- • Bulb 3: 22,000 hours
- • Bulb 4: 28,000 hours
- • Bulb 5: 35,000 hours
Calculation:
Sum = 25,000 + 30,000 + 22,000 + 28,000 + 35,000 = 140,000 hours
MTTF = 140,000 ÷ 5 = 28,000 hours
Median Life = 28,000 × ln(2) ≈ 19,408 hours
Frequently Asked Questions
When should I use MTTF instead of MTBF?
Use MTTF when the component cannot be repaired and must be replaced after failure (e.g., bearings, seals, light bulbs, fuses, semiconductor chips). Use MTBF when the system can be repaired and returned to service (e.g., machines, vehicles, servers).
Why is Reliability at MTTF only 36.8%?
Under the exponential distribution, R(MTTF) = e⁻¹ ≈ 0.368. This means about 63.2% of units have already failed by the MTTF. The MTTF is the mean (average), not the median — it is pulled higher by units that last longer than average.
What is median life (B50)?
Median life (B50) is the time at which 50% of units have failed. For the exponential distribution, B50 = MTTF × ln(2) ≈ 0.693 × MTTF. The median is often more practical than the mean for planning purposes.
Can MTTF and MTBF have the same value?
Numerically, yes — the formulas are identical. The difference is conceptual: MTTF is for items that are discarded after failure, while MTBF is for items that are repaired. Using the correct term shows you understand the system you are analyzing.