Core Reliability

MTTF Calculator

Mean time to failure for non-repairable items — from total operating time or individual lifetimes — with the MTTF vs MTBF distinction explained.

MTTF = Σtᵢ ÷ n

Input Method

Time Unit

Total Operating Time (hours)

Number of Units Failed

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 units that failed

From individual lifetimes

MTTF = (t₁ + t₂ + … + tₙ) ÷ n

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

A manufacturer tests 5 LED bulbs until failure and records their lifetimes:

Bulb	Lifetime
1	25,000 h
2	30,000 h
3	22,000 h
4	28,000 h
5	35,000 h

Sum of lifetimes

25,000 + 30,000 + 22,000 + 28,000 + 35,000 = 140,000 h

MTTF

140,000 ÷ 5 = 28,000 h

If lifetimes followed an exponential distribution, the median life would be MTTF × ln 2 ≈ 19,408 h. LEDs wear out rather than fail randomly, so for this sample the observed median (28,000 h) is the more realistic figure — a reminder to check the distribution before relying on exponential formulas.

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.