The short product of two power series is the meaningful part of the product of these objects, i.e. $\sum_{i+j < n} a_ib_j x^{i+j}$. In \cite{Mulders00}, Mulders gives an algorithm to compute a short product faster than the full product in the case of Karatsuba's multiplication \cite{KaOf62}. This algorithm works by selecting a cutoff point $k$ and performing a full $k\times k$ product and two $(n-k)\times (n-k)$ short products recursively. Mulders also gives a heuristically optimal cutoff point $\beta n$. In this paper, we determine the optimal cutoff point in Mulders' algorithm. We also give a slightly more general description of Mulders' method.