Although inductors oppose changes in current in an electronic circuit, they don't oppose all changes equally. All inductors present more opposition to fast current changes than they do to slower changes, or put another way, inductors oppose current changes in higher-frequency signals more than they do in lower-frequency signals.

The degree to which an inductor opposes current change at a particular frequency is called the inductor's reactance. Inductive reactance is measured in ohms, just like resistance, and can be calculated with the following formula:


Here, the symbol XL represents the inductive reactance in ohms, f represents the frequency of the signal in hertz (cycles per second), and L equals the inductance in henrys. Oh, and π is that magic mathematical constant you learned about in high school, whose value is approximately 3.14.

For example, suppose you want to know the reactance of a 1 mH inductor to a 60 Hz sine wave (not coincidentally, the frequency of household power). The math looks like this:


Thus, a 1 mH inductor has a reactance of about a third of an ohm at 60 Hz.

How much reactance does the same inductor have at 20 kHz? Much more:


Increase the frequency to 100 MHz and see how much resistance the inductor has:


At low frequencies, inductors are much more likely to let current pass than at high frequencies. This characteristic can be exploited to create circuits that block frequencies above or below certain values.