What Is Reactance and How Is It Calculated?
Reactance is the opposition that an inductor or capacitor presents to alternating current, measured in ohms (Ω) and dependent on frequency. Unlike resistance, reactance stores and returns energy rather than dissipating it as heat, and it shifts the current 90° out of phase with the voltage.
The two reactance formulas are: X_L = 2πfL for an inductor and X_C = 1 / (2πfC) for a capacitor, where f is frequency in hertz, L is inductance in henries and C is capacitance in farads. In a series RLC circuit, total impedance is Z = √(R² + (X_L − X_C)²) and the phase angle is φ = arctan((X_L − X_C) / R).
According to the International Electrotechnical Commission (IEC), mains power is distributed at either 50 Hz or 60 Hz worldwide, so a 100 mH inductor presents about 31.4 Ω of reactance at 50 Hz and 37.7 Ω at 60 Hz.
How to Use This Reactance Calculator
Pick a series or parallel circuit, enter the frequency, and fill in whichever components your circuit contains. Results update instantly:
- Frequency (f): The AC signal frequency in Hz, kHz or MHz. Reactance changes with every frequency, so this is required.
- Inductance (L): Optional. Choose nH, µH, mH or H. Leave blank if there is no inductor.
- Capacitance (C): Optional. Choose pF, nF, µF or mF. Leave blank if there is no capacitor.
- Resistance (R): Optional, in ohms. Needed for a full impedance and phase calculation.
The formulas are the same worldwide and independent of unit system. Use 50 Hz for mains circuits in the UK, EU, India and Australia, 60 Hz for the US, Canada and much of Latin America, and kHz–MHz for audio and radio work.
Reactance Values at Common Frequencies
Reactance moves in opposite directions: inductive reactance rises with frequency while capacitive reactance falls, as the table shows for a 10 mH inductor and a 1 µF capacitor.
| Frequency | X_L (10 mH) | X_C (1 µF) |
|---|---|---|
| 50 Hz | 3.14 Ω | 3,183 Ω |
| 60 Hz | 3.77 Ω | 2,653 Ω |
| 1 kHz | 62.8 Ω | 159 Ω |
| 10 kHz | 628 Ω | 15.9 Ω |
The two reactances are equal at the resonant frequency f₀ = 1 / (2π√(LC)), about 1.59 kHz for this pair, where a series circuit reaches minimum impedance equal to R alone.
Frequently Asked Questions
What is the formula for capacitive reactance?
Capacitive reactance is X_C = 1 / (2πfC), where f is frequency in hertz and C is capacitance in farads. For example, a 1 µF capacitor has a reactance of about 159 Ω at 1 kHz, falling as frequency rises.
What is the difference between reactance and impedance?
Reactance is only the frequency-dependent opposition from inductors and capacitors. Impedance is the total opposition to AC, combining resistance and net reactance as a vector: Z = √(R² + X²). Both are measured in ohms.
How accurate is this reactance calculator?
It applies the ideal-component formulas exactly. Real capacitors and inductors have tolerance (often ±5–20%), equivalent series resistance and parasitic effects that grow at high frequency, so measured values near self-resonance can differ from the calculated result.
Why is inductive reactance positive and capacitive reactance negative?
By convention, inductors make current lag the voltage by 90° (+jX_L), while capacitors make current lead by 90° (−jX_C). Because they have opposite signs, they partly cancel in a series circuit, and fully cancel at resonance.