Report

Help us improve this tool

Amdahl's Law Calculator

Calculate theoretical speedup from Amdahl's law using parallelizable task proportion, speedup factor, and original execution time.

O M T

What Is Amdahl's Law?

Amdahl's law estimates the maximum speedup you can achieve by improving only part of a task. Even large speedups on a parallelizable fraction are limited by the sequential portion that cannot be sped up.

Amdahl's Law Formula

$$S = \frac{1}{1 - p + \frac{p}{s}}$$ $$T_{new} = \frac{T_{old}}{S}$$

Where \(p\) is the parallelizable fraction, \(s\) is the speedup factor applied to that fraction, and \(S\) is the overall speedup.

Maximum Possible Speedup

$$S_{max} = \frac{1}{1 - p}$$

Related tools: Productivity Calculator and Binary Calculator.

Frequently Asked Questions

Why does speedup plateau even with many processors?

The sequential fraction sets a hard ceiling. If 30% of the task cannot be parallelized, speedup cannot exceed about 3.33× no matter how many cores you add.

What should p represent?

p is the fraction of total execution time that benefits from parallelization or other performance improvements.

Does this predict real-world performance exactly?

No. It is a theoretical upper bound. Memory, I/O, and coordination overhead can reduce actual gains.

When should I use Amdahl's law?

Use it when deciding whether investing in parallel hardware or optimizing a hot code path is worth the cost.