Choose a Conversion
Formulas
Power ratio
dB = 10 log10(P2 / P1)
Voltage ratio
dB = 20 log10(V2 / V1)
The voltage formula assumes the two voltages are measured across the same impedance.
Worked Example
A power increase from 1 W to 100 W is a ratio of 100:
10 log10(100 / 1) = 20 dB
A 20 dB power gain means 100 times more power, while a 20 dB voltage gain means 10 times more voltage at equal impedance.
Use and Limits
Use decibels to add gains and losses without repeatedly multiplying ratios. Positive dB indicates gain; negative dB indicates loss.
dB is a ratio, not an absolute power unit. dBm is absolute power referenced to 1 mW.