A simple method is proposed to calibrate phase shift modulators used in phase-shifting interferometry for algorithms requiring π/2 phase shifts. The method provides a convenient in situ calibration without any change in experimental setup for a wide range of phase shift modulators such as piezoelectric translators (PZTs), Pockels cells, etc. The calibration by the proposed method is fast, efficient, and accurate. The performance of the new method is demonstrated by calibration of a Pockels cell in a phase-stepped speckle interferometer. The results obtained from the new calibration method are found to agree to within 2% of those from a previously described method for the calibration of nonlinear phase modulators.