Divide the first signal's power by the second signal's power to find the ratio of the two signals. For instance, if signal A has a power of 20 mW and signal B has a power of 5 mW: 20/5 = 4.
Take the log of the the ratio of the signals by pressing the log button on the scientific calculator. For instance: log 4 = 0.602.
Multiply this answer by 10 to find the decibels. For the example: 0.602 x 10 = 6 decibels (dB).
Determine if the decibel reading reflects a loss or gain of power by looking at signal A and signal B. Record a loss if signal A had a greater value than signal B, and a gain if signal B had the greater number. For the example, since the first signal (signal A) measured more than signal B, the result indicated a loss of 6 decibels (dB).