The paper describes a new algorithm for measuring frequency at a power system bus. The algorithm is based on the Newton-Raphson iterative method using the digitized voltage samples at a relay location. The mathematical development of the algorithm is presented and the effects of key parameters that affect the performance of the algorithm are discussed. The algorithm is encompassing the Newton-Raphson method, which is very common for load flow studies. The algorithm shows a high measurement accuracy over a wide range of frequency changes. To demonstrate the efficiency of the proposed algorithm, results of a numerical experiment are presented.