Power Gain

When referring to measurements of power or intensity, a ratio can be expressed in decibels (G) by evaluating ten times the base-10 logarithm of the ratio of the measured quantity (Pout) to the reference level (Pin).

G=10log left( frac{P_{out}}{P_{in}} right)

P_{in}=frac{P_{out}}{10^frac{G}{10}}

P_{out}=10^frac{G}{10}P_{in}

More info: http://en.wikipedia.org/wiki/Decibel#Power