Let $f$ be the fraction of the computation (in terms of time) that is parallelizabl$e$. $P$ the number of processors in the system, and $s_p$ the speed up achievable in comparison with sequential execution – then the $s_p$ can be calculated using the relation:
speed up = time without parallelism / time with parallelism
= 1/ 1-f + f/P
// Correct me if I am wrong!