Power Factor Calculator
Enter the real and apparent power of an AC circuit and read off the power factor, the phase angle, and the reactive power — the three numbers that describe how efficiently it draws current.
Two inputs, three answers
Give the calculator the real power (W) and apparent power (VA) and it returns the power factor, the phase angle, and the reactive power at once.
Apparent power is never smaller
The apparent power S must be at least the real power P — it contains it. If you enter S below P the calculation has no physical answer.
What is a power factor calculator?
Real and apparent power in, the power triangle out
In an AC circuit, not all the power the supply delivers does useful work. The power factor calculator turns two measurements — the real power (the watts doing work) and the apparent power (the volt-amperes actually supplied) — into the numbers that describe that gap: the power factor itself, the phase angle between current and voltage, and the reactive power that flows back and forth without doing anything. A power factor of 1 means every supplied volt-ampere does work; a lower value means current is being drawn that the load never converts to useful output. Utilities care because a poor power factor means more current for the same useful power, so this tool is the everyday starting point for motors, transformers, and power-factor correction.
Enter the real power in watts and the apparent power in volt-amperes to get the power factor, phase angle, and reactive power instantly.
The three quantities form a right triangle — the "power triangle" — with apparent power as the hypotenuse, real power along the base, and reactive power as the vertical side.
PF = P ÷ SThe phase angle is the angle of that triangle, φ = arccos(P / S), and the reactive power is the third side, Q = √(S² − P²), found by the Pythagorean theorem. A power factor of 0.8 corresponds to a phase angle of about 36.87° and, for an 800 W / 1000 VA load, a reactive power of 600 VAR.
Suppose a motor draws 1000 VA of apparent power but only 800 W of real power.
Power factor
PF = 800 ÷ 1000 = 0.8 — 80% of the supplied power does useful work.
Phase angle
φ = arccos(0.8) ≈ 36.8699° — how far the current lags the voltage.
Reactive power
Q = √(1000² − 800²) = √360,000 = 600 VAR — the power that shuttles back and forth.
The power factor is the headline number, and it is easiest to read as a percentage of efficiency: 0.8 means 80% of every volt-ampere the supply delivers turns into useful watts, while the remaining 20% is tied up shuttling energy in and out of the load's magnetic or capacitive fields. A value near 1.0 is ideal — purely resistive loads like heaters sit there — and values that drift toward 0 signal a heavily inductive load such as a lightly loaded motor. The phase angle (about 36.87° for PF = 0.8) is the same information in angular form: it is the lag between the current and voltage waveforms, and it climbs to 90° as the power factor falls to zero. The reactive power Q (600 VAR here) is the side of the triangle that does no work but still loads the wires and transformer — it is why a low power factor matters in practice: the same 800 W of useful output forces 1000 VA of current through the system, raising losses and demanding bigger conductors. Correcting the power factor — usually by adding capacitors to cancel inductive reactive power — shrinks Q, pulls the phase angle toward zero, and brings the apparent power back down toward the real power.
The power triangle is exact for steady sinusoidal AC, but real circuits add wrinkles.
Sinusoidal, single-frequency circuits only
PF = P / S and Q = √(S² − P²) describe the displacement power factor of a clean, single-frequency sinusoidal circuit. With non-linear loads — switching power supplies, LED drivers, variable-speed drives — current harmonics add a distortion component, and the true power factor is lower than this geometric value alone suggests. The result also gives only the magnitude of the angle, not whether the load is inductive (lagging) or capacitive (leading); you need to know the load type to say which. Always enter apparent power in VA and real power in W, and keep S ≥ P — an apparent power below the real power is physically impossible.