The reactance calculator above converts a capacitance or an inductance into the opposition it presents at a given frequency. It reports capacitive reactance and inductive reactance separately, the net reactance when the two sit in series, and the frequency at which the two are equal and cancel each other out. Every figure is in ohms, because reactance shares its unit with resistance even though it behaves nothing like it.
Arb Digital builds free tools that stay inside one clear question. This page handles reactance alone, with no resistance in the circuit. The moment a resistor joins the loop the quantity you want is impedance rather than reactance, and the site's RLC impedance calculator combines all three into a magnitude and a phase angle. Use this page to understand the components, and that one to analyse the circuit.
What This Reactance Calculator Does
A resistor opposes current the same way at every frequency. A capacitor and an inductor do not, and reactance is the number that describes how strongly each one opposes an alternating current at a particular frequency. The two behave in opposite directions. Capacitive reactance falls as frequency rises, so a capacitor blocks direct current completely and passes high frequencies easily. Inductive reactance rises with frequency, so an inductor is nearly a plain wire at direct current and increasingly obstructive as frequency climbs.
That opposition is why the same component does completely different jobs in different places. A capacitor in series with a signal path is a high-pass filter because it blocks the low frequencies. The same capacitor across a supply rail is a decoupling capacitor because it presents a low reactance to the high-frequency noise and shunts it away. Neither behaviour is a property of the capacitor on its own; both fall out of the reactance at the frequencies involved.
The net reactance in the hero result is the series combination, X = XL − XC. Subtraction rather than addition is not a convention chosen for convenience: the two reactances shift current and voltage in opposite directions in time, so their contributions genuinely oppose. A positive net reactance means the pair behaves inductively overall, a negative one means capacitively, and zero means the two have cancelled and the series pair is at resonance.
How to Use It
- Enter the operating frequency. This is the frequency of the signal you care about, not the resonant frequency of the circuit, which the calculator works out for you.
- Enter the capacitance and the inductance. Use the unit selectors rather than typing exponents, which is where most arithmetic errors on this kind of calculation come from.
- Leave a component at zero if it is not there. Zero capacitance is treated as an open circuit and zero inductance as a plain wire, which is what those limits physically mean.
- Add an applied voltage if you want a current. The tool divides the voltage by the magnitude of the net reactance. Leave it at zero and the reactance results still stand on their own.
- Read the cancellation frequency. It tells you where this pair of components stops opposing current and starts resonating, which is often the most useful thing on the page.
The Formulas: How Reactance Is Calculated
Both definitions come from the same place. OpenStax University Physics Volume 2, section 15.2 on simple AC circuits, defines capacitive reactance as XC = 1/(ωC) and inductive reactance as XL = ωL, describing each as the opposition of that component to a change in current. With angular frequency ω = 2πf, those become XC = 1/(2πfC) and XL = 2πfL.
Setting the two equal and solving for frequency gives the point at which they cancel: 2πfL = 1/(2πfC) rearranges to f0 = 1/(2π√(LC)). That is the series resonant frequency. Reactance carries the ohm as its unit, which NIST Special Publication 811, the guide to the use of the SI, defines as one volt per ampere. It depends only on the product of inductance and capacitance, not on either one separately — which is why a large inductor with a small capacitor and a small inductor with a large capacitor can resonate at exactly the same frequency while behaving very differently either side of it.
Work the defaults. At 200 Hz with 800 µF, the capacitive reactance is 1 ÷ (2π × 200 × 0.0008) = 0.9947 Ω. With 3 mH at the same frequency, the inductive reactance is 2π × 200 × 0.003 = 3.7699 Ω. The net series reactance is 3.7699 − 0.9947 = 2.7752 Ω, positive and therefore inductive. The cancellation frequency is 1 ÷ (2π√(0.003 × 0.0008)) = 102.7 Hz, comfortably below 200 Hz, which is consistent with the pair reading inductive at the operating frequency: above resonance the inductor wins.
Why Reactance Is in Ohms but Is Not Resistance
The shared unit misleads people constantly. Reactance is measured in ohms because it is a ratio of volts to amps, exactly like resistance, and you can use it in Ohm's law to get a current. What it does not share is the energy behaviour. A resistor turns electrical energy into heat, permanently. An ideal capacitor or inductor stores energy and gives all of it back, so the average power it consumes over a complete cycle is zero.
The mechanism is phase. In a resistor, current and voltage rise and fall together, so their product is positive throughout the cycle and energy always flows one way. In a capacitor the current leads the voltage by a quarter cycle, and in an inductor it lags by a quarter cycle. Either way, for half of each cycle the instantaneous power is negative, meaning energy is flowing back out of the component into the source. Over a full cycle the two halves cancel exactly.
The practical consequence is that reactive current is real current with real consequences. It heats the wiring, it loads the transformer, and it occupies capacity in the supply, all without delivering useful work. That is the entire basis of power factor as a billing and engineering concern, and it is why reactive components have to be counted in a supply calculation even though they consume nothing on average. The Ohm's law calculator handles the resistive case where none of this applies.
The Ideal Component and the Real One
Every number this page produces assumes a perfect component, and real ones diverge in ways that matter at the extremes. A real capacitor has equivalent series resistance and its own small series inductance from its leads and internal structure. Above a certain frequency that stray inductance dominates and the capacitor stops behaving as a capacitor at all: its impedance stops falling and starts rising. This self-resonant frequency is the reason a decoupling network often uses several capacitors of widely different values rather than one large one.
A real inductor has winding resistance and inter-winding capacitance, and it too self-resonates, above which it behaves capacitively. It also has a core that can saturate, at which point the inductance itself falls and the reactance falls with it, which is a failure mode a linear calculation cannot warn you about. Neither self-resonant frequency is a general figure; both come from the specific part's data sheet.
There is a second, subtler assumption. These formulas describe steady-state sinusoidal operation, meaning the circuit has settled and the signal is a single clean sine wave. They say nothing about what happens when a switch closes, which is a transient governed by time constants rather than reactances, and the site's capacitor charge time calculator covers that case. Applying a reactance to a square wave or a switching edge is also a misuse, because those signals are sums of many frequencies and each component sees a different reactance.
How This Differs From the Adjacent Circuit Tools
The boundary in one sentence: this page gives the reactance of a capacitor and an inductor at a stated frequency with no resistance present, while the RLC impedance calculator combines resistance and reactance into the total impedance magnitude and phase angle of an actual circuit.
The filter cutoff calculator answers a different question again: it finds the frequency at which a resistor-capacitor or resistor-inductor pair reaches the half-power point, which is where reactance equals resistance rather than where two reactances cancel. The capacitor charge time calculator handles the direct-current transient, where reactance does not apply at all. For the stored energy in each component, see the capacitor energy calculator and the inductor energy calculator, and for capacitance from plate geometry the capacitance calculator.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Adding the two reactances — they oppose each other, so the series total is the inductive value minus the capacitive one, and adding them can overstate the answer several times over.
- Entering the resonant frequency as the operating frequency — the tool derives resonance for you, and feeding it back in reports a net reactance of zero that means nothing about your actual signal.
- Typing exponents by hand — use the unit selectors, because a picofarad mistaken for a nanofarad is a factor of a thousand in the answer.
- Treating reactance as a heat source — an ideal reactance dissipates no average power, and the heating in a real part comes from its parasitic resistance instead.
- Applying these results above a part's self-resonant frequency — beyond that point a capacitor acts inductive and an inductor acts capacitive, and the formulas here describe neither.
Related Free Tools From Arb Digital
For a complete circuit with resistance included, use the RLC impedance calculator, and for filter corner frequencies the filter cutoff calculator. The capacitor energy calculator and the inductor energy calculator cover stored energy, the capacitance calculator derives capacitance from geometry, and the capacitor charge time calculator handles direct-current transients. The Ohm's law calculator covers the purely resistive case. Rescale values with the capacitance converter or the inductance converter, and browse the full free online tools hub for everything else.
Frequently Asked Questions
Because it is a ratio of volts to amps, which is what the ohm measures. The difference is energetic rather than dimensional: a resistor converts energy to heat permanently, while an ideal reactance stores energy and returns all of it to the source, consuming zero average power over a full cycle.
Because they shift current relative to voltage in opposite directions in time. A capacitor makes current lead by a quarter cycle and an inductor makes it lag by a quarter cycle, so their effects genuinely oppose and the series total is the inductive reactance minus the capacitive one.
That the capacitor dominates at the frequency entered, so the series pair behaves capacitively overall. A positive value means the inductor dominates and the pair behaves inductively. The sign flips as you cross the cancellation frequency.
Infinite. Capacitive reactance is inversely proportional to frequency, so as frequency approaches zero the reactance grows without limit, which is the mathematical statement that a capacitor blocks direct current. An inductor at direct current has zero reactance and behaves as a plain wire.
It depends only on the product of inductance and capacitance, so a large inductor with a small capacitor resonates at the same frequency as a small inductor with a large capacitor. What differs is how sharply the circuit behaves either side of resonance, which the product alone does not tell you.
Not directly. These formulas describe steady-state sinusoidal operation at a single frequency. A square wave is a sum of many frequency components, each of which sees a different reactance, so a single number cannot describe how the component responds to it.
Because real capacitors have lead and internal inductance, and above their self-resonant frequency that stray inductance dominates so the part behaves inductively. Real inductors self-resonate too and turn capacitive above it. Both frequencies come from the specific part's data sheet.
It does no useful work on average, but it is real current that heats conductors and occupies supply capacity, which is why it is counted in power factor and supply calculations. Reactive components consume nothing on average yet still impose a real load on the wiring feeding them.
This tool is provided for educational and estimating use only. It models ideal components in steady-state sinusoidal operation and ignores equivalent series resistance, self-resonance, core saturation and every other real-part effect. It is not electrical design advice, and any work on mains-connected equipment must be designed and carried out by a suitably qualified person under the rules applicable in your jurisdiction.
