dBm is a unit of power. The abbreviation dBm means power ratio in decibel (dB) with reference to the power 1 milliwatt (mW).

Decibel (dB) is a dimensionless unit which quantifies the ratio or difference between two values. In other words, the increase or decrease in a signal. In the case of dBm, one value is our measured power and the other is 1 mW.

Decibels are useful when working with signals as signal strengths vary logarithmically, not linearly. The dB scale is logarithmic which means it produces simple numbers for large-scale variations in signals.

Doubling (or halving) the power level adds (or subtracts) 3 dB to the power level which corresponds to a 50% gain or reduction (Young, n.d.).

Power in dBmPower mW
-30 dBm0.001 mW
-50 dBm0.00001
-70 dBm0.0000001
-90 dBm0.000000001

The formula to convert from power x in dBm to power P mW is

P = 1mW * 10^(x/10)

Reference: Young, M. (n.d.). Understanding Decibels and Their Use in Radio Systems. [online] YDI wireless. [Accessed 28 Jun. 2023].