Mutation Score Calculator

Work out mutation score instantly with clear inputs, formula shown and shareable results.

About Mutation Score Calculator

Mutation testing injects small behavioural changes and checks whether the suite notices. The score is killed mutants over viable ones, excluding equivalent mutants that cannot change observable behaviour. The gap against line coverage is the interesting number: a large gap means tests execute the code without asserting on it, which is the failure mode line coverage cannot see.

Formula

Mutation score

viable = generated - equivalent; score = killed / viable; gap = line coverage - mutation score

Worked example

With the default values below, Mutation Score Calculator returns Mutation score of 70.27%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Mutants generatedminimum 1: 2,400
Mutants killed by testsminimum 0: 1,560
Equivalent or unviable mutantsminimum 0: 180
Line coverage for comparisonaccepts 0–100 %: 85 %

Results

Mutation score: 70.27%
Surviving mutants: 660
Viable mutants tested against: 2,220
Gap between line coverage and mutation score: 14.73%

Step by step

1Values used
2Mutation score
3Mutation score= 70.27
4Surviving mutants= 660
5Viable mutants tested against= 2,220
6Gap between line coverage and mutation score= 14.73

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 2 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 is mutation testing so slow?

The suite must run once per mutant. Practical tools mitigate this with coverage-guided selection so only tests touching the mutated line are run.

What mutation score is good?

Above 70 percent on core business logic is strong. Applying it to the whole codebase is usually too slow, so scope it to the modules where correctness matters most.