An absolute calibration procedure for periodic microwave phase shifters is described. Increments on the phase shifter are compared by substitution with a reproducible phase-shift step of initially unknown value. The periodic nature of the phase shifter provides a reference phase shift of 360/spl deg/, and alternative methods are outlined for the derivation of the correction curve from the step measurements. It is shown that the accuracy of the calibration is governed by the precision and reproducibility of the phase shifter under test.