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Capacitive and inductive reactance calculator

Capacitors and coils oppose AC with reactance. It's measured in ohms, like resistance, but it changes with frequency: a capacitor's falls as the frequency rises, and a coil's rises. Pick a part and a frequency to get its reactance, the current a voltage pushes through it, and the impedance with a resistor in series.

1k = 1,000 Hz. Mains is 50 or 60 Hz.

µ or u = micro, n = nano, p = pico.

Peak or RMS: the current comes out the same kind.

Leave empty for the part on its own.

Capacitive reactance, X = 1 ÷ (2π f C)

159 Ω
Current, I = V ÷ X
6.28 mA
At 10 kHz, ten times f
15.9 Ω
A coil with the same reactance
25.3 mH

The formulas

XC = 1 over 2π f C XL = 2π f L

f is in hertz, C in farads and L in henries. Reactance goes into Ohm's law just like resistance, I = V ÷ X, as long as you use peak with peak, or RMS with RMS. With a resistor in series, the two add at right angles, like the sides of a right-angled triangle:

|Z| = √(R² + X²)

A worked example

A 1 µF capacitor at 1 kHz:

XC = 1 over 2π × 1000 × 10⁻⁶ ≈ 159 Ω

So 1 V across it pushes 1 ÷ 159 ≈ 6.3 mA. A coil with the same 159 Ω at 1 kHz is 159 ÷ (2π × 1000) ≈ 25 mH. Here's how a 1 µF capacitor and a 10 mH coil compare:

FrequencyXC (1 µF)XL (10 mH)
100 Hz1.59 kΩ6.28 Ω
1 kHz159 Ω62.8 Ω
10 kHz15.9 Ω628 Ω

That's why a speaker's tweeter has a capacitor in series: it blocks the bass.

Unlike a resistor, an ideal capacitor or coil takes no average power. Energy flows in for a quarter of each cycle and back out the next.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“Reactance” is lesson 2 of 6 in Oscillation Coast, Electronics World 4. Free with an account. Lessons unlock in order, so you’ll do the 22 before it first.

Or start from the very first lesson, no account needed

The ideas behind it

Questions people ask

What is capacitive reactance?

A capacitor's opposition to AC, in ohms: X_C = 1 ÷ (2π × f × C). It falls as the frequency rises, which is why a capacitor blocks DC but passes high frequencies.

What is inductive reactance?

A coil's opposition to AC, in ohms: X_L = 2π × f × L. It rises with frequency, so a coil passes DC and low frequencies and holds back fast ones. Power supplies use coils, called chokes, to block noise.

What is the difference between reactance and impedance?

Reactance comes from capacitors and coils, resistance from resistors, and impedance is the two together. In series they add at right angles: |Z| = √(R² + X²). So 1 kΩ with 1 kΩ of reactance is 1.41 kΩ, not 2 kΩ.

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