Amdahl's Law Calculator (Parallel Speedup)
Enter the parallel share of the work and the number of processors to see the speedup limit.
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Amdahl's Law Calculator (Parallel Speedup) by TheCalcTools
How the Amdahl's Law Calculator (Parallel Speedup) works
Amdahl’s law says the serial part of a program limits the overall speedup no matter how many processors you add: speedup = 1 ÷ ((1 − p) + p ÷ n).
If 90% of the work runs in parallel, the speedup can never exceed 10×. Gustafson’s law takes the more optimistic view that larger machines are used for larger problems.
The formula
Worked example
Using the default values shown in the calculator:
| Parallelisable share of the work | 90 % |
|---|---|
| Processors or cores | 16 |
- 90% parallel on 16 cores: 1 ÷ (0.1 + 0.9 ÷ 16) = 6.40×.
- Even with unlimited cores the maximum is 10.00×.
Result:
| Speedup (Amdahl) | 6.40 |
|---|---|
| Parallel efficiency | 40.0% |
| Limit with unlimited processors | 10.00× |
| Scaled speedup (Gustafson) | 14.50 |
Frequently asked questions
Why does adding cores give diminishing returns?
Each extra core only speeds up the parallel part, so the fixed serial part takes a growing share of the time.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.