A new seven-sample algorithm has been developed which is particularly useful for phase evaluation in systems with non-sinusoidal periodic waveforms. The algorithm performs well in systems with significant amounts of second, third, fourth, and sixth harmonic content and in the presence of phase shift errors. The algorithm is characterized by an equi-spaced symmetrical sampling pattern in which the end samples are one full period apart. Principal features of the algorithm in terms of the Fourier theory of phase-shifting are presented. Performance of the seven-sample algorithm is outlined and compared with several conventional algorithms.