The** resonant frequency** condition arises in the series circuit when the inductive reactance is equal to the capacitive reactance. If the supply frequency is changed the value of X_{L} = 2πfL and X_{C} = 1/2πfC is also changed.

When the frequency increases, the value of X_{L} increases, whereas the value of X_{C} decreases. Similarly, when the frequency decreases, the value of X_{L} decreases and the value of X_{C} increases.

Thus, to obtain the condition of series Resonance, the frequency is adjusted to f_{r}, point P as shown in the curve below. At point P when (XL = XC) the resonant frequency condition is obtained.

At series resonance, when XL = XC

Where fr is the resonant frequency in hertz when inductance L is measured in Henry and capacitance C in Farads.