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Walk-through

How to Use This Calculator

3 steps

1

Enter frequency and capacitance

On the Capacitive Reactance tab, enter the AC frequency (Hz, kHz, or MHz) and the capacitor's rated capacitance (F, µF, nF, or pF). The calculator updates instantly as you type.

2

Read the capacitive reactance

The result shows Xc in ohms — the opposition the capacitor presents to AC current at that frequency. Add an optional inductance value to see the inductive reactance at the same frequency in the detail line, for a quick side-by-side comparison.

3

Compare XL or solve backward

Switch to the Inductive Reactance tab to compute XL = 2πfL directly (with an optional Xc comparison), or to the Solve tab to pick Xc, frequency, or capacitance as the unknown and let the calculator rearrange Xc = 1/(2πfC) to solve for it.

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Reference

Formula & Methodology

3 formulas

Capacitive reactance

Xc = 1 / (2πfC)

Capacitive reactance Xc (in ohms) is the opposition a capacitor presents to alternating current. It depends on the signal frequency f (in hertz) and the capacitance C (in farads) — reactance falls as frequency or capacitance rises, so a capacitor passes high frequencies more easily than low ones.

Inductive reactance

XL = 2πfL

Inductive reactance XL (in ohms) is the opposition an inductor presents to alternating current, for comparison. Unlike capacitive reactance, it rises with frequency and inductance L (in henries) — inductors resist high frequencies more than low ones, the opposite behavior of a capacitor.

Solving backward

f = 1 / (2πC·Xc); C = 1 / (2πf·Xc)

Rearranging Xc = 1/(2πfC) for the other two variables lets you find the frequency at which a known capacitor reaches a target reactance, or the capacitance needed to reach a target reactance at a known frequency.

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Glossary

Key Terms Explained

6 terms

Capacitive reactance (Xc) The frequency-dependent opposition a capacitor presents to AC current, measured in ohms (Ω). Xc = 1/(2πfC) — it decreases as frequency or capacitance increases, and approaches infinity as frequency approaches zero (a capacitor blocks DC).

Inductive reactance (XL) The frequency-dependent opposition an inductor presents to AC current, measured in ohms (Ω). XL = 2πfL — it increases with frequency and inductance, the opposite trend of capacitive reactance.

Frequency (f) The number of AC cycles per second, measured in hertz (Hz). Line power in the US is 60 Hz; audio signals typically range from 20 Hz to 20 kHz; RF circuits can run into the MHz or GHz range.

Capacitance (C) A capacitor's ability to store electric charge per unit voltage, measured in farads (F). Practical capacitors are usually specified in microfarads (µF), nanofarads (nF), or picofarads (pF).

Impedance (Z) The total AC opposition a circuit element presents, combining resistance and reactance as a complex quantity: Z = R + jX. For a pure capacitor, impedance is purely reactive (Z = -jXc); for a pure inductor, Z = jXL.

AC circuit A circuit driven by an alternating (sinusoidal) voltage or current source, as opposed to a DC circuit with constant polarity. Reactance only exists in AC circuits — capacitors and inductors behave differently than resistors because their opposition to current depends on how fast the signal changes.

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Scenarios

Real-World Examples

3 worked examples

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Technician checking a power-supply filter capacitor

60 Hz line frequency, 10 µF capacitor

Frequency 60 Hz Capacitance 10 µF

Xc = 1 / (2π × 60 × 0.00001) ≈ 265.3 Ω. At the standard US line frequency, a 10 µF capacitor presents about 265 Ω of opposition to the 60 Hz component of the signal.

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Audio engineer comparing a crossover capacitor at two frequencies

Same 4.7 µF capacitor at 100 Hz vs. 10 kHz

Capacitance 4.7 µF Frequency A / B 100 Hz / 10,000 Hz

At 100 Hz, Xc ≈ 338.6 Ω; at 10 kHz, Xc ≈ 3.39 Ω — a 100x increase in frequency drops the reactance by the same factor. This is why capacitors are used as high-pass elements in audio crossovers: they pass high frequencies with little opposition while blocking low ones.

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Student comparing capacitive and inductive reactance

60 Hz circuit with a 10 µF capacitor and a 100 mH inductor

Frequency 60 Hz Capacitance 10 µF Inductance 100 mH

Xc ≈ 265.3 Ω while XL = 2π × 60 × 0.1 ≈ 37.7 Ω. The two reactances move in opposite directions as frequency changes — they would be equal (resonance) at a much higher frequency, around 159 Hz for this L-C pair.

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Deep Dive

Understanding the Capacitive Reactance Calculator

3 chapters · 3 min read

Capacitive reactance describes how much a capacitor opposes the flow of alternating current at a given frequency — unlike a resistor, that opposition changes depending on how fast the signal alternates. This calculator computes capacitive reactance from frequency and capacitance, compares it against inductive reactance, and can solve backward for the frequency or capacitance needed to hit a target reactance.

How the Capacitive Reactance Calculator works

On the Capacitive Reactance tab, the calculator applies Xc = 1/(2πfC), where f is frequency in hertz and C is capacitance in farads. On the Inductive Reactance tab, it applies the analogous XL = 2πfL for a comparison component. On the Solve tab, it rearranges the same relationship — f = 1/(2πC·Xc) or C = 1/(2πf·Xc) — to find whichever variable you mark as unknown. These are the standard reactance formulas used throughout AC circuit analysis, filter design, and impedance matching.

Inputs and what they mean

Frequency (f) is the AC signal or line frequency, entered in Hz, kHz, or MHz. Capacitance (C) is the capacitor's rated value, entered in F, µF, nF, or pF — most practical capacitors fall in the µF-to-pF range. Inductance (L), where relevant, is entered in H, mH, or µH for the comparison inductor. On the Solve tab, capacitive reactance (Xc) itself becomes an input when you're solving for frequency or capacitance instead. Because Xc scales inversely with both frequency and capacitance, doubling either one halves the reactance.

Limits and edge cases

This calculator models an ideal capacitor and inductor — it ignores equivalent series resistance (ESR), parasitic inductance in real capacitors, and temperature or voltage-dependent capacitance drift, all of which shift real-world reactance slightly from the ideal formula. At zero frequency (DC), Xc is mathematically infinite — a capacitor fully blocks DC current once charged — so the calculator requires frequency and capacitance to both be greater than zero. For circuits with both capacitance and inductance together, remember that Xc and XL move in opposite directions with frequency; the point where they're equal is the circuit's resonant frequency, which this calculator does not compute directly (see the LC Resonance calculator for that).

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Questions

Frequently Asked Questions

6 questions

What is the capacitive reactance formula?+

Capacitive reactance is Xc = 1/(2πfC), where f is frequency in hertz and C is capacitance in farads. The result is in ohms.

What happens to reactance at higher frequency?+

Capacitive reactance falls as frequency rises — the two are inversely proportional. A capacitor that strongly opposes a low-frequency signal will barely oppose a much higher-frequency one, which is why capacitors are used to pass high frequencies and block low ones.

What is inductive reactance?+

Inductive reactance is XL = 2πfL, where L is inductance in henries. Unlike capacitive reactance, it rises with frequency — inductors oppose high-frequency signals more than low-frequency ones.

What units does capacitive reactance use?+

Capacitive and inductive reactance are both measured in ohms (Ω), the same unit as resistance. Frequency is in hertz (Hz), capacitance in farads (F, µF, nF, or pF), and inductance in henries (H, mH, or µH).

What is the capacitive reactance at DC (0 Hz)?+

At 0 Hz, Xc is mathematically infinite — an ideal capacitor fully blocks direct current once it charges up, which is why capacitors are used to filter out DC and pass only AC signals.

Can this calculator solve for capacitance instead of reactance?+

Yes — use the Solve tab and choose "Capacitance (C)" as the unknown. Enter a target reactance and a known frequency, and the calculator rearranges Xc = 1/(2πfC) to return the capacitance needed.

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Capacitive Reactance Calculator

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