To calculate the value of a resistor, you will need the following information: the desired voltage across the resistor, the current flowing through the resistor, and the desired resistance. Once you have this information, you can use Ohm’s Law, which states that voltage equals current multiplied by resistance (V = I x R), to calculate the value of the resistor. For example, if you want a voltage of 12V across a resistor with a current of 0.5A and a resistance of 24 ohms, you would calculate the value of the resistor as follows: V = I x R (12V = 0.5A x R), R = V / I = 12V / 0.5A = 24 ohms.