dBm

dBm is a unit of power used in radio engineering that expresses power levels on a logarithmic scale relative to one milliwatt. It is defined by the equation P(dBm) = 10 log10(P / 1 mW), where P is the power in milliwatts. Equivalently, P(W) = 10^((P(dBm) − 30)/10) in watts. The dBm scale is convenient for comparing very large or very small powers in communications systems.

The dBm unit is related to the decibel-relative-to-watt unit (dBW) by a simple offset: dBm = dBW + 30, since 1 W equals 1000 mW. Conversely, dBW = dBm − 30. When converting from voltage or field quantities to power, the assumed load impedance (commonly 50 ohms in RF work) must be specified, because power depends on both voltage and the load.

Common values illustrate the scale: 0 dBm equals 1 mW, 10 dBm equals 10 mW, 20 dBm equals 100 mW, and 30 dBm equals 1 W. Negative values indicate powers below 1 mW, for example −30 dBm is 1 µW.

dBm is widely used to specify transmitted power, received signal strength (RSSI), and link budgets in wireless systems. It provides a compact way to represent power levels across large dynamic ranges. Because it is a logarithmic unit, small changes in dBm correspond to substantial proportional changes in power.