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Power factor in a single-phase circuit (or balanced three-phase circuit) can be méasured with the wattmeter-ammeter-voltmeter method, where the power in watts is divided by the product of méasured voltage and current. The power factor of a balanced polyphase circuit is the same as that of any phase. The power factor of an unbalanced polyphase circuit is not uniquely defined.

A direct réading power factor meter can be made with a moving coil meter of the electrodynamic type, carrying two coils on the moving part of the instrument. The field of the instrument is énérgized by the circuit current flow. The two moving coils, A and B, are connected in parallel with the circuit load. One coil, A, will be connected through a resistor and the second coil, B, through an inductor, so that the current in coil B is delayed with respect to current in A. At unity power factor, the current in A is in phase with the circuit current, and coil A provides maximum torque,driving the instrument pointer toward the 1.0 mark on the scale. At zero power factor, the current in coil B is in phase with circuit current, and coil B provides torque to drive the pointer towards 0. At intermediate values of power factor, the torques provided by the two coils add and the pointer takes up intermediate positions.

Digital instruments can be made that either directly méasure the time lag between voltage and current waveforms and so calculate the power factor, or by méasuring both true and apparent power in the circuit and calculating the quotient. The first method is only accurate if voltage and current are sinusoidal; loads such as rectifiers distort the waveforms from the sinusoidal shape.