Decibel (dB) Converter
Convert between dB and power or voltage ratios, and between dBm, watts and volts.
Formulas
For power, dB = 10·log10(P2/P1). For voltage, dB = 20·log10(V2/V1). So 3 dB is about twice the power and 6 dB is about twice the voltage. dBm is absolute power referenced to 1 mW: 0 dBm is 1 mW and 30 dBm is 1 W.
Frequently asked questions
How much is 3 dB?
About twice the power (×1.995), or about 1.41 times the voltage. −3 dB is half the power, which is why filter cutoffs are defined at −3 dB.
What is the difference between dB and dBm?
dB is a ratio between two values and has no unit. dBm is an absolute power referenced to 1 mW, so 20 dBm is exactly 100 mW.