Amdahl Speedup Calculator

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

About Amdahl Speedup Calculator

Amdahl's law caps speedup at 1/(1-p) no matter how many processors are added: with 85% parallel code the ceiling is 6.7x. Adding a per-core overhead term shows why speedup often peaks and then declines as cores are added.

Formula

Amdahl's law

S = 1 / ((1 - p) + p/N); ceiling = 1 / (1 - p)

Worked example

With the default values below, Amdahl Speedup Calculator returns Achieved speedup of 3.53. Change any input above and every figure here is recalculated in your browser.

Inputs used

Parallelisable fraction: 85 %
Processors: 16 cores
Parallel overhead per core: 0.5 %

Results

Achieved speedup: 3.53 x
Ideal Amdahl speedup: 4.92 x
Maximum speedup with infinite cores: 6.67 x
Parallel efficiency: 22.08% %

Step by step

1Values used
2Amdahl's law
3Achieved speedup= 3.53 x
4Ideal Amdahl speedup= 4.92 x
5Maximum speedup with infinite cores= 6.67 x
6Parallel efficiency= 22.08 %

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

Why does speedup plateau so quickly?

The serial fraction becomes the whole runtime. At 85% parallel, 16 cores already achieve 5.9 of the 6.7 maximum, so further cores are nearly worthless.

Is Amdahl's law the whole story?

No. Gustafson's law argues problem size grows with machine size, which gives a far more optimistic scaling picture for many real workloads.