Reactance Plotter

Log-log sweep of a component's reactance versus frequency. Flip between capacitor ( $X_C = 1/(2\pi f C)$ , slope −1) and inductor ( $X_L = 2\pi f L$ , slope +1) modes. Drag the frequency marker to read reactance at any frequency; the component slider slides the whole line up (smaller value) or down (bigger value) without changing its slope.

Mode

Capacitance C

10.0 µF

Mark frequency

50.0 Hz

At the marked frequency

f50.0 HzC10.0 µFX_C318 Ω

Quick sanity checks

10 µF cap on 50 Hz mains (capacitor mode): $X_C$ ≈ 318 Ω. Raise f to 5 kHz and $X_C$ drops by 100× to ~3.18 Ω.
100 mH choke on 50 Hz mains (inductor mode): $X_L$ ≈ 31.4 Ω. Raise f to 20 kHz and $X_L$ jumps by 400× to ~12.6 kΩ — exactly what a switching-supply output filter sees.
Decade-per-decade rule in either mode: every 10× change in frequency is a 10× change in reactance (opposite direction for $X_C$ and $X_L$ ). Park the marker, drag it one decade, read the new value — the straight line makes this a ruler.

Reactance and impedance

Reactance is the frequency-dependent opposition to current flow in capacitors and inductors. Unlike resistance, reactance does not dissipate energy — it stores and returns it each cycle.

Capacitor reactance

X_C = 1 / (2πfC) — falls with frequency. A capacitor is an open circuit at DC (f = 0) and approaches a short at very high frequencies. Doubles when C halves; halves when f doubles.

Inductor reactance

X_L = 2πfL — rises with frequency. An inductor is a short at DC and an open at very high frequencies. Doubles when L doubles or f doubles.

Series resonance

In a series LC circuit, X_L and X_C cancel at resonance: f₀ = 1 / (2π√(LC)). Impedance is minimum (= R only) at f₀, making the circuit pass that frequency — the basis of band-pass filters and tuned circuits.

Learn more → Reactance — Learn

Quick experiments

Watch a capacitor block DC. Sweep a 1 µF capacitor from 1 Hz upward. At 1 Hz its reactance is about 159 kΩ; at 1 kHz it is 159 Ω. Extrapolate to 0 Hz and reactance goes to infinity — that is what 'blocks DC' means numerically.
Watch an inductor do the opposite. Plot a 10 mH inductor. At 1 Hz it is 0.063 Ω, essentially a wire; at 100 kHz it is 6.3 kΩ. Same component, five orders of magnitude apart.
Find where the two curves cross. Plot 1 µF and 10 mH together. They intersect near 1.6 kHz — the resonant frequency, where the reactances are equal and cancel in a series circuit.
See the decade rule. Every tenfold rise in frequency divides capacitive reactance by 10 and multiplies inductive reactance by 10. On log-log axes both are straight lines, sloping opposite ways.
Size a coupling capacitor. For a 10 kΩ input impedance, pick C so its reactance is well under 1 kΩ at the lowest frequency of interest. At 20 Hz that needs roughly 10 µF.

Formula reference

Capacitive reactance: $X_C = \frac{1}{2\pi f C}$
1 µF at 1 kHz gives about 159 Ω. Falls as frequency rises.
Inductive reactance: $X_L = 2\pi f L$
10 mH at 1 kHz gives about 63 Ω. Rises with frequency.
Resonance — where the two are equal: $f_0 = \frac{1}{2\pi\sqrt{LC}}$
Set the two reactances equal and solve. 10 mH with 1 µF resonates near 1.59 kHz.
Net series reactance: $X = X_L - X_C$
Positive is inductive, negative is capacitive, zero is resonance.

Symbol	Meaning	Unit
$X_C$	Capacitive reactance	Ω
$X_L$	Inductive reactance	Ω
$f$	Frequency	Hz
$f_0$	Resonant frequency	Hz

Common mistakes

Treating reactance as if it dissipated power.
Reactance is measured in ohms but stores and returns energy rather than converting it to heat. Ideal reactive elements dissipate zero average power — only the resistive part does.
Mixing up which element blocks which frequencies.
Capacitors block low frequencies and pass high ones; inductors do the reverse. If a result contradicts that, the formula has been inverted.
Adding reactance and resistance arithmetically.
They are 90° apart, so they add as vectors: Z = √(R² + X²). With R = 30 Ω and X = 40 Ω the impedance is 50 Ω, not 70 Ω.
Forgetting the 2π.
The formulas use ω = 2πf, not f. Leaving out 2π scales the answer by 6.28 — a mistake that hides easily because the result still looks reasonable.
Assuming a real capacitor stays capacitive at any frequency.
Lead inductance means every real capacitor self-resonates and turns inductive above that point. It is why datasheets specify a self-resonant frequency and why decoupling caps are chosen by package size.

Frequently asked questions

What is reactance?

Reactance is the opposition a capacitor or inductor presents to alternating current, measured in ohms. Unlike resistance it depends on frequency and it stores energy rather than dissipating it, so it shifts the phase between voltage and current instead of turning power into heat.

How do I calculate capacitive reactance?

Capacitive reactance is Xc = 1 / (2 pi f C). It falls as frequency rises, so a capacitor blocks DC and passes high frequencies. A 1 microfarad capacitor at 1 kHz has a reactance of about 159 ohms.

How do I calculate inductive reactance?

Inductive reactance is Xl = 2 pi f L. It rises with frequency, so an inductor passes DC and blocks high frequencies. A 10 millihenry inductor at 1 kHz has a reactance of about 63 ohms.

Why is capacitive reactance treated as negative?

In phasor form capacitive reactance is written as -j times Xc and inductive as +j times Xl, because capacitor current leads voltage by 90 degrees while inductor current lags by 90 degrees. The opposite signs let the two cancel, which is exactly what happens at resonance.