Voltage Gain

When referring to measurements of amplitude, when impedances at input and output are equal, a ratio can be expressed in decibels (G) by evaluating twenty times the base-10 algorithm of the ratio of the measured quantity (Vout) to the reference level (Vin).

G=20log left( frac{V_{out}}{V_{in}} right)



More info: http://en.wikipedia.org/wiki/Decibel#Amplitude.2C_voltage_and_current