Gustafson Speedup Calculator

Work out gustafson speedup instantly with clear inputs, formula shown and shareable results.

About Gustafson Speedup Calculator

Gustafson's law assumes the problem grows with the machine, giving S = s + p x N — linear in processor count rather than asymptotically capped. It is the right model for weak scaling, where a bigger machine is used to solve a bigger problem in the same time.

Formula

Gustafson's law

S = s + (1 - s) x N, where s is the serial fraction of the scaled workload

Worked example

With the default values below, Gustafson Speedup Calculator returns Scaled speedup of 58.96. Change any input above and every figure here is recalculated in your browser.

Inputs used

Serial fraction: 8 %
Processors: 64 cores
Problem scaling with cores: 1 x

Results

Scaled speedup: 58.96 x
Amdahl speedup for comparison: 10.60 x
Scaled efficiency: 92.13% %
Parallel work fraction: 92.00% %

Step by step

1Values used
2Gustafson's law
3Scaled speedup= 58.96 x
4Amdahl speedup for comparison= 10.60 x
5Scaled efficiency= 92.13 %
6Parallel work fraction= 92.00 %

Assumptions and limits

The result is derived only from the 3 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

When does Gustafson apply rather than Amdahl?

When the workload scales with the machine — larger simulations, finer meshes, bigger models. Amdahl applies to fixed-size problems (strong scaling).

Which is more realistic for HPC?

Gustafson, generally. Nobody buys a bigger cluster to run the same small problem faster; they run a larger problem.