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Test Coverage (Fault) Calculator

Work out test coverage (fault) instantly with clear inputs, formula shown and shareable results.

About Test Coverage (Fault) Calculator

Fault coverage is detected faults over testable faults. The Williams-Brown model links it to defect escapes: DL = 1 - (1 - Y)^(1-T), which shows why coverage must reach 99%+ for automotive parts — at 95% coverage and 2% incoming defects, thousands of parts per million escape.

Formula

Defect level

DL = 1 - (1 - defect level)^(1 - coverage); coverage = detected / testable faults

Worked example

With the default values below, Test Coverage (Fault) Calculator returns Fault coverage of 96.8153%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total testable faults
2,400,000 faults
Faults detected
2,280,000 faults
Untestable faults
45,000 faults
Incoming defect level
2 %

Results

Fault coverage
96.8153% %
Test efficiency
96.8750% %
Defect escape rate
643.2 DPPM
Undetected testable faults
75,000 faults

Step by step

  1. 1Values used
  2. 2Defect level
  3. 3Fault coverage= 96.8153 %
  4. 4Test efficiency= 96.8750 %
  5. 5Defect escape rate= 643.2 DPPM
  6. 6Undetected testable faults= 75,000 faults

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why chase the last 1% of coverage?

Because escapes fall roughly exponentially with coverage. Going from 98% to 99.5% can cut DPPM by a factor of four.

What makes faults untestable?

Redundant logic, unreachable states and structures excluded by design-for-test constraints. They are excluded from the coverage denominator but must be justified.

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