Test Parallelisation Calculator

Work out test parallelisation instantly with clear inputs, formula shown and shareable results.

About Test Parallelisation Calculator

Test parallelisation is bounded below by the single longest test, because no split can finish faster than its slowest indivisible unit. Once total work divided by workers falls below that duration, extra workers only add setup cost. The maximum useful worker count is therefore total time divided by the longest test.

Formula

Parallel test time

wall clock = max(total / workers, longest single test) + setup per worker; max useful workers = total / longest test

Worked example

With the default values below, Test Parallelisation Calculator returns Wall-clock test time of 287. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total test time on one workerminimum 1 s: 1,450 s
Parallel workersaccepts 1–256: 6
Setup time per workerminimum 0 s: 45 s
Longest single testminimum 0.1 s: 90 s

Results

Wall-clock test time: 287 s
Speedup: 5.06 x
Workers beyond which nothing improves: 16
Limiting factor: Limited by total work divided across workers

Step by step

1Values used
2Parallel test time
3Wall-clock test time= 287 s
4Speedup= 5.06 x
5Workers beyond which nothing improves= 16
6Limiting factor= Limited by total work divided across workers

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 does one slow test cap everything?

Because tests are the unit of scheduling. A 90-second test occupies one worker for 90 seconds no matter how many other workers are idle.

How should tests be distributed?

By recorded duration, packing workers to equal time rather than equal count. Most runners support timing-based splitting from a previous run's report.