dBm

dBm, short for decibel-milliwatts, is a logarithmic unit expressing absolute radio-frequency 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, quantities that differ by ten or more orders of magnitude. By definition, 0 dBm equals exactly 1 mW; positive values are more than a milliwatt, negative values less.

The logarithmic shorthand

The conversion is simple: 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, 14 dBm is about 25 mW, and -30 dBm is one microwatt. Received signals are minuscule compared with transmitted ones, which is why the strength readings on a mesh radio are negative dBm values — often -80 to -120 dBm — with numbers nearer zero meaning a stronger signal. A change from -100 dBm to -90 dBm is not a small improvement; it is ten times more received power.

Why operators think in dBm

Working in decibels turns multiplication into addition, which is what makes a link budget tractable on the back of a napkin: start with transmit power in dBm, add antenna gains in dBi, subtract feedline and path losses in dB, and the result is the received power in dBm. Compare that against the receiver's sensitivity and you know whether the link closes, with how much margin, before you climb a single roof. Regulatory transmit limits are quoted both ways — the 25 mW common in the European 868 MHz band is roughly 14 dBm, and North American ISM-band limits are likewise stated as EIRP figures in dBm that include antenna gain, which is why bolting a high-gain antenna onto a maximum-power transmitter can quietly push a station out of compliance.

dBm on the mesh bench

For anyone building sovereign off-grid comms, dBm is the working vocabulary. A LoRa radio's headline feature is receive sensitivity: its chirp modulation lets it decode signals far below -120 dBm at high spreading factors, well under the noise floor of conventional narrowband receivers, which is how a node transmitting a fraction of a watt covers kilometers. When a Meshtastic node reports the RSSI and SNR of a neighbor, the RSSI figure is received power in dBm — and watching it change as you move an antenna is the cheapest site-survey tool there is. A few dB gained by raising an antenna above a roofline or swapping lossy coax often matters more than any settings change, because 3 dB of recovered loss is a doubling of effective power.

Common reference points

A handful of values cover most field situations:

• +30 dBm = 1 W — high-power transmitter territory
• +20 dBm = 100 mW — a typical maximum for many license-free radios
• +14 dBm ≈ 25 mW — the familiar European 868 MHz limit
• 0 dBm = 1 mW — the reference itself
• -80 dBm — a comfortably strong received mesh signal
• -110 to -125 dBm — the weak-signal region where LoRa's modulation still decodes but margins are thin

Memorize the 10 dB and 3 dB rules and you can interpolate everything between these anchors without a calculator.

One caution: dBm is an absolute power unit, while plain dB expresses a ratio. Gains and losses are in dB; power levels are in dBm; you add dB to dBm, but adding two dBm figures together is meaningless. Keeping that distinction straight is half of RF literacy. dBm underpins nearly every other radio metric on this site — see RSSI for received power measurement, link budget for adding and subtracting across a path, and attenuation (RF) for the loss terms themselves.