quality:correction, linguistic:paraphrasing
Voltage gain

The power gain can be calculated using voltage instead of power using Joule's first law P = V 2 / R {\displaystyle P=V^{2}/R}; the formula is:

In many cases, the input impedance R in {\displaystyle R_{\text{in}}} and output impedance R out {\displaystyle R_{\text{out}}} are equal, so the above equation can be simplified to:

This simplified formula, the 20 log rule, is used to calculate a voltage gain in decibels and is equivalent to a power gain if and only if the impedances at input and output are equal.