dBm

dBm, short for decibel-milliwatts, is a logarithmic unit expressing absolute radio power referenced to one milliwatt. It is the default unit of RF engineering because a single compact number can describe both a transmitter's output and a receiver's noise floor, which differ by many orders of magnitude. By definition, 0 dBm equals exactly 1 mW.

The logarithmic shorthand

The conversion is power in dBm equals ten times the base-ten logarithm of power in milliwatts. Two rules of thumb make field math easy: every 10 dB step multiplies or divides power by ten, and every 3 dB step roughly doubles or halves it. So 20 dBm is 100 mW, 30 dBm is 1 W, and -30 dBm is one microwatt. Because received signals are minuscule, the strength readings on a mesh radio are negative dBm values, with numbers nearer zero meaning more power.

Why operators think in dBm

Working in dBm turns multiplication into addition: antenna gains and path losses simply add and subtract, which is what makes a link budget tractable on the back of a napkin. Regulatory transmit limits, often quoted in milliwatts such as the 25 mW common in the 868 MHz band, are routinely restated in dBm as roughly 14 dBm.

dBm underpins nearly every other RF metric here. See RSSI for received power in dBm, Link Budget for adding and subtracting it across a path, and Attenuation (RF) for the loss terms.