Method: For a balanced three-phase AC system using line-to-line voltage: S = √3 × V × I P = √3 × V × I × PF For a single-phase AC system: S = V × I P = V × I × PF where S is apparent power, P is real power, V is RMS voltage, I is RMS current and PF is power factor. Reactive power magnitude is calculated from: Q = √(S² − P²) The calculator reports the magnitude of reactive power. Determining whether reactive power is inductive or capacitive requires knowing whether the power factor is lagging or leading. Three-phase calculations assume a reasonably balanced system. For significantly unbalanced loads, phase-by-phase measurements should be used instead.