A voltage divider is a simple circuit which takes a voltage source, Vin, and divides the voltage across two resistors in series, R1 and R2.
The divided voltage or the output voltage, Vout, is then measured across one of the resistors, either R1 or R2.
To calculate the output voltage across R1 use the following equation: Vout = (R1*Vin)/(R1+R2). Likewise to measure the output voltage across R2 use the equation: Vout = (R2*Vin)/(R1+R2).
If a circuit has more than two resistors, an equivalent circuit can be created to reduce the number of resistors down to two. Or a new voltage divider equation can be created. For more information on how to do this, read about Kirchhoff's Voltage Laws.