Resonance occurs when the reactance of an inductor balances the reactance of a capacitor at some given frequency. In such a resonant circuit where it is in series resonance, the current will be maximum and offering minimum impedance. In parallel resonant circuits the opposite is true.
The formula for resonance is:

2 * pi * f * L = 1 / (2 * pi * f * C)
where: 2 * pi = 6.2832; f = frequency in hertz L = inductance in Henries and C = capacitance in Farads 
Leading to: 

f = 1 / [2 * pi (sqrt LC)]
where: 2 * pi = 6.2832; f = frequency in hertz L = inductance in Henries and C = capacitance in Farads
A particularly simpler formula using a factor of 25330.3 for radio frequencies is:

LC = 25330.3 / f 2
where: f = frequency in Megahertz (Mhz) L = inductance in microhenries (uH) and C = capacitance in picofarads (pF)
By using simple algebra we can determine:

LC = 25330.3 / f 2 and L = 25330.3 / f 2 C and C = 25330.3 / f 2 L
Impedance at Resonance
In a series resonant circuit the impedance is at its lowest for the resonant frequency whereas in a parallel resonant circuit the impedance is at its greatest for the resonant frequency. See figure 1.
"For a series circuit at resonance, frequencies becoming far removed from resonance see an ever increasing impedance. For a parallel circuit at resonance, frequencies becoming far removed from resonance see an ever decreasing impedance".

where: 
X_C = capacitive reactance in ohms  
f = frequency in hertz 
C = capacitance in farads 
Notice the use of 2 X PI Squared which is 6.28.  This factor is used in almost all AC equations, especially for Amateur Radio use.  Here again, the speed of light deals with the time it takes for part or whole of a sine wave to complete. 

Unlike resistance, inductive reactance is usually undesirable in a circuit. Whereas resistance is created by a resistor to achieve some effect, reactance is an unfortunate by-product of certain electrical components. There are two basic types of reactance: Capacitive reactance and inductive reactance. Appropriately enough, capacitive reactance is created by capacitors, while inductive reactance is created by inductors. Using either of these device types in an AC circuit will introduce some reactance. Like resistance, reactance is expressed in ohms, and it behaves in much the same way as resistance, in the sense that it tends to restrict the flow of current through a circuit.

The formula for calculating inductive reactance is as follows:

XL = 2*pi*f*L

XL is the inductive reactance. X is the general electrical symbol for "reactance", and L is the symbol for "inductance" or "inductor", so put them together and you get the reactance of an inductor.

pi is, as you probably guessed, the famous ratio of a circle's circumference to its diameter, to wit: 3.14, etc.

f is the frequency of the AC flowing through the circuit.

L is the inductance of the inductor, in henries.

So you see, when you muliply 2 by pi, by the frequency of the AC, by the inductance of the inductor, the resultant value is the inductive reactance of the circuit.