Impedance Impedance, Z = V I Resistance, R = V I V = voltage in volts (V) I = current in amps (A) Z = impedance in ohms ( ) R = resistance in ohms ( ) Impedance (symbol Z) is a measure of the overall opposition of a circuit to current, in other words: how much the circuit impedes the flow of current. It is like resistance, but it also takes into account the effects of capacitance and inductance. Impedance is measured in ohms, symbol . Impedance is more complex than resistance because the effects of capacitance and inductance vary with the frequency of the current passing through the circuit and this means impedance varies with frequency ! The effect of resistance is constant regardless of frequency. The term 'impedance' is often used (quite correctly) for simple circuits which have no capacitance or inductance - for example to refer to their 'input impedance' or 'output impedance'. This can seem confusing i...