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MTBF Test Time Calculator

Reliability Demonstration MIL-HDBK-338 Analysis Statistical Testing
Free MTBF Test Time Calculator

Calculate the total operating accumulated test time (T) required to demonstrate a specified Mean Time Between Failures (MTBF) target at a designated confidence level, assuming an exponential failure distribution (constant failure rate).

Test Calculation Inputs
Required Test Demonstration
916
Total Unit-Hours Accumulated (T)
183.2
Test Duration Per Unit (Hours)
0.916x
Test Time Multiplier (T / MTBF)

Testing 5 units for 183.2 hours each (accumulating 916 total hours) with 0 allowable failures demonstrates an MTBF of 1,000 hours at a 60% confidence level.

Statistical Demonstration Models (MIL-HDBK-338)

Test duration calculations rely on the Chi-Square (χ2) lower confidence limit for an exponential distribution:

Type I Test • Time-Truncated
Fixed Test Duration Model

Testing stops at a predetermined time T regardless of failure count. Degrees of freedom incorporate the unobserved failure buffer (2r + 2).

T = MTBF · χ2(1 - α,  2r + 2) 2
Degrees of Freedom (df) = 2r + 2
Type II Test • Failure-Terminated
Fixed Failure Count Model

Testing stops exactly when the r-th failure occurs. Degrees of freedom correspond strictly to observed failure occurrences (2r).

T = MTBF · χ2(1 - α,  2r) 2
Degrees of Freedom (df) = 2r  (min df = 2 for 0 failures)
T Total Unit-Hours
MTBF Target Operating Life
r Allowable Failures
1 - α Confidence Level
Need Complete Component-Level MTBF Predictions?

Switch from basic demonstration test planning to multi-standard failure rate prediction with ALD software.

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Component-Level MTBF Analysis

Calculate component failure rates across MIL-HDBK-217F, Telcordia SR-332, and FIDES standards.

Open Advanced MTBF Tool