**Impedance** (measured in Ohms) is the measure of the resistance to current that a circuit presents over a given voltage, or the ratio between the rms values of current and voltage. In DC circuits, impedance is the same as resistance.

Impedance is a complex number, with resistance being the real part and reactance being the imaginary part. In general, the formula for impedance is

- $ \ Z = R + jX = R + j (X_L - X_C) $

With *R* being the resistance, *j* the imaginary unit (not *i* to avoid confusion with current), *X _{L}* being the inductive reactance, and

*X*being the capacitive reactance.

_{C}