Reactance Calculator: XL, XC, Impedance and Phase Angle

How to use it

Type the supply Frequency, for example 50 for 50 Hz.
Type the parts of the circuit into Resistance, Inductance and Capacitance. Leave a box empty when that part is not there.
Read Inductive reactance and Capacitive reactance at that frequency, then the Series impedance and Parallel impedance with their phase angles.
Open Working, step by step to check each figure by hand.

The formulas

A coil and a capacitor oppose alternating current in a way that depends on frequency. That opposition in ohms is the reactance: XL is the reactance of the inductor and XC that of the capacitor. In a series circuit the reactances add to the resistance as vectors; in a parallel circuit the admittances (1 ÷ Z) of the branches add.

XL = 2πfL
XC = 1 ÷ (2πfC)

Series:    Z = R + jX,  X = XL − XC
           |Z| = √(R² + X²),  θ = arctan(X ÷ R)

Parallel:  Y = 1/R + j(2πfC − 1 ÷ (2πfL))
           |Z| = 1 ÷ |Y|,  θ = −arg(Y)

f frequency (Hz)   L inductance (H)   C capacitance (F)   R resistance (Ω)

A positive phase angle means the voltage leads the current, which is what an inductive circuit does. A negative angle means the capacitor dominates.
Series and parallel are two circuits built from the same parts: the same R, L and C give a different impedance in each.

What the calculator does not cover

Circuits that mix series and parallel parts, or that hold more than one resistor, inductor or capacitor in one branch. Enter the equivalent values, or apply the impedance rules branch by branch.
The losses of real parts: the resistance of a coil's wire, or the equivalent series resistance of a capacitor, is part of R only when you enter it.
Frequency effects in real parts, such as the core losses of an inductor and the tolerance of a capacitor.

Worked example: 50 Hz with R 10 Ω, L 100 mH and C 100 µF

The inductive reactance is XL = 2π × 50 × 0.1 = 31.42 Ω, and the capacitive reactance is XC = 1 ÷ (2π × 50 × 0.0001) = 31.83 Ω.

In series the net reactance is X = 31.42 − 31.83 = −0.415 Ω, so |Z| = √(10² + 0.415²) = 10.01 Ω at a phase angle of −2.38°. The capacitor slightly outweighs the coil, so the circuit is a little capacitive.

In parallel the resistor branch has an admittance of 1/10 = 0.1 S, and the net susceptance of the two reactive branches is only −0.415 mS. So |Z| = 10 Ω at +0.24°, very close to the resistor on its own.

Frequently asked questions

What is reactance measured in?

Reactance is measured in ohms, like resistance. It is the part of the impedance that a capacitor or inductor stores and gives back, instead of turning the energy into heat.

Why does capacitive reactance fall as the frequency rises?

A capacitor passes more current at a higher frequency, because it has less time to charge in each cycle: XC = 1 ÷ (2πfC) gets smaller as f grows. Inductive reactance does the opposite, since XL = 2πfL grows with frequency.

Why does the series phase angle come out negative?

A negative angle means the capacitive reactance is larger than the inductive reactance, so the current leads the voltage. The sign follows the sign of X = XL − XC.

Can I type 4k7 or 100µ in the boxes?

Yes. Depending on the box, 4k7 is read as 4.7 kΩ and 100µ as 100 µF. A unit typed after the number, such as 100µF or 50Hz, is ignored.

Why is the parallel impedance different from the series one?

In parallel the branch currents add, so the impedance is the inverse of the sum of the branch admittances. The same parts make different current paths, so they give a different impedance.

What does a resistor of zero ohms do in the parallel result?

A parallel resistor of zero ohms shorts the circuit, so the parallel impedance is zero and that result is not shown. Leave the resistance box empty if there is no resistor.

Sources

Figures and text last checked on Tuesday, November 10, 2026.

Result

Working, step by step