Simple power analysis is a common technique to attack software implementations, especially in the realm of public-key cryptography. An effective countermeasure to protect an implementation is to ensure constant (worst-case) runtime. In this paper we show how to modify an algorithm by Kaliski to compute the Montgomery inverse such that it can compute both the classical and Montgomery modular inverse in constant time. We demonstrate the effectiveness by comparing it to the approach based on Fermat’s little theorem as used in the current simple power analysis resistant implementations in cryptography. Our implementation on the popular 32-bit ARM platform highlights the practical benefits of this algorithm.