Inductive reactance is calculated with a straightforward formula: XL = 2πfL, where f is the frequency of the AC signal in hertz and L is the inductance in henries. The result, XL, is measured in ohms, just like resistance. If you know the frequency and the inductance value, you can find the inductive reactance in seconds with a calculator.
The Formula and Its Variables
The core equation is:
XL = 2πfL
- XL = inductive reactance in ohms (Ω)
- 2π = approximately 6.28 (the number of radians in one full cycle of an AC waveform)
- f = frequency of the AC signal in hertz (Hz)
- L = inductance in henries (H)
You might also see this written as XL = ωL, where ω (omega) is the angular frequency. Angular frequency is just 2π multiplied by the regular frequency, so the two versions are identical.
A Worked Example
Say you have a 10 µH inductor in a circuit running at 1 MHz, a common scenario in power electronics or RF filtering. Before plugging anything in, you need to convert units so everything is in henries and hertz. 10 µH equals 0.00001 H (move the decimal six places left), and 1 MHz equals 1,000,000 Hz.
Now apply the formula: XL = 6.28 × 1,000,000 × 0.00001 = 62.8 ohms. That means the inductor opposes current flow with 62.8 ohms of reactance at that frequency.
If you changed the frequency to 100 kHz (100,000 Hz) with the same inductor, the reactance drops to 6.28 ohms. At 10 MHz, it climbs to 628 ohms. The relationship is perfectly linear: double the frequency, double the reactance.
Getting Your Unit Conversions Right
The most common mistake when calculating inductive reactance is forgetting to convert inductance into henries. Real-world inductors are rarely labeled in full henries. Standard values range from about 1 µH up to 100 mH, and they follow a standardized series of preferred values (1.0, 1.2, 1.5, 2.2, 3.3, 4.7, and so on).
Here are the conversions you’ll use most often:
- 1 millihenry (mH) = 0.001 H
- 1 microhenry (µH) = 0.000001 H
- 1 millihenry = 1,000 microhenries
So a 4.7 mH inductor becomes 0.0047 H in the formula, and a 2.2 µH inductor becomes 0.0000022 H. Similarly, convert kilohertz to hertz by multiplying by 1,000, and megahertz by multiplying by 1,000,000.
Why Frequency Changes Everything
Inductive reactance is directly proportional to frequency. At 0 Hz (pure DC), the formula gives zero ohms, meaning an inductor acts like a plain wire for direct current. As frequency rises, so does the opposition to current flow. High frequencies are impeded the most.
This is the opposite of how capacitors behave, which block DC and pass high-frequency signals freely. That contrast is why inductors and capacitors are often paired in filter circuits: the inductor blocks high frequencies while the capacitor blocks low ones.
The physical reason is that higher frequencies mean the current is changing faster. An inductor resists changes in current by generating a voltage proportional to how quickly the current is changing. Faster changes produce more opposition.
How Reactance Differs From Resistance
Both reactance and resistance are measured in ohms, and both limit current flow. The critical difference is what happens to the energy. A resistor converts electrical energy into heat. An inductor stores energy temporarily in a magnetic field and then releases it back into the circuit. Reactance resists current without dissipating power.
This also creates a timing difference. In a purely inductive circuit, the voltage leads the current by 90 degrees, meaning voltage peaks a quarter of a cycle before the current does. In a purely resistive circuit, voltage and current peak at the same moment. This phase shift matters when you’re calculating total impedance in circuits that contain both resistors and inductors, because you can’t simply add resistance and reactance together. Instead, impedance is calculated as the square root of R² + XL².
Multiple Inductors in a Circuit
When inductors are connected in series (one after another), their individual reactances add up directly, just like resistors in series. If one inductor has 50 ohms of reactance and another has 30 ohms, the total inductive reactance is 80 ohms.
In parallel configurations, the math mirrors parallel resistance: 1/Xtotal = 1/X1 + 1/X2. If your circuit also contains capacitors, the net reactance is the difference between inductive and capacitive reactance (X = XL − XC), since the two types oppose each other.
Measuring Inductance When You Don’t Know It
If you’re working with an unmarked inductor or need to verify a component’s value, an LCR meter is the standard tool. It works by applying a small test signal, measuring the resulting voltage and current, and then calculating the inductance from those readings.
A few practical tips for accurate measurements: LCR meters have two modes, series and parallel. Use series mode for low-impedance components (roughly 100 Ω or less), which includes most small inductors. Use parallel mode for high-impedance components above about 10 kΩ. Before measuring, run the meter’s built-in open and short correction routines, which compensate for stray capacitance and resistance in the test leads. For inductors with ferrite or iron cores, use constant-current mode, since the inductance of core-based inductors can shift depending on how much current flows through them.
Once you have the inductance value, plug it into XL = 2πfL at whatever frequency your circuit operates, and you have the reactance.