What dBm Actually Measures

The Live Packets stream shows an SNR number on every packet. Datasheets quote transmit power in dBm. Antenna specs list gain in dBi. Link budget calculations add and subtract across all of these in one chain. They all speak the same language — and dBm is where that language starts.

The decibel problem it solves

Radio work spans an enormous range of power levels. A LoRa node transmits at roughly +20 dBm — 100 milliwatts, which is 0.1 watts. That same transmission, received by a node several kilometers away through terrain and buildings, might arrive at −90 dBm — about 1 nanowatt, or 0.000000001 watts. The ratio between those two levels is 100,000,000:1.

Working with numbers that span eight orders of magnitude in raw watts is unwieldy. Linear arithmetic on them is error-prone. The decibel solves this by working in logarithms: it converts multiplication into addition, which turns that 100,000,000:1 ratio into a tidy “−110 dB.” Engineers have been doing this since the 1920s because it makes chain calculations — transmitter power through cable through antenna through propagation through receiver — a sequence of additions and subtractions rather than a chain of multiplications.

dBm anchors this logarithmic scale to a concrete reference: 1 milliwatt.

The definition and the key numbers

dBm = 10 × log₁₀(power in mW / 1 mW)

You do not need to compute this from scratch for day-to-day work. A handful of reference points are all you need:

dBm	Power
0 dBm	1 mW
+3 dBm	~2 mW
+10 dBm	10 mW
+20 dBm	100 mW
+30 dBm	1 W
−10 dBm	0.1 mW
−30 dBm	1 µW
−60 dBm	1 nW
−90 dBm	1 pW

The two rules that matter most in practice:

+3 dB ≈ double the power. An amplifier with 3 dB gain doubles whatever signal it receives. A cable with 3 dB loss halves it. These round numbers compound: 6 dB is 4× power, 9 dB is 8×, 10 dB is 10×.

+10 dB = 10× the power. This makes it easy to reason about orders of magnitude. A signal at −80 dBm is 10 times stronger than one at −90 dBm, and 100 times stronger than one at −100 dBm.

dB vs. dBm: the distinction that trips people

dB (without a suffix) is a relative ratio — it describes a change, a gain, or a loss, with no fixed reference. A cable with 3 dB loss reduces whatever signal passes through it by half. That statement is true regardless of the absolute power level.

dBm is absolute — it anchors the measurement to 1 milliwatt. −90 dBm is a specific amount of power (1 picowatt) that you can measure with a power meter.

The arithmetic rule: dBm + dB = dBm. Apply a gain (dB) to an absolute level (dBm) and you get a new absolute level (dBm). Subtract cable loss (dB) from transmit power (dBm) and you get the power at the antenna port (dBm). This is the backbone of every link budget.

What you cannot do: add two dBm values. Two separate signals at −80 dBm do not sum to −160 dBm — that is not how power addition works in the logarithmic domain. (The correct answer, if you actually need to add two equal powers, is approximately −77 dBm: −80 dBm + 3 dB.) But for antenna and link calculations, you almost never need to add two absolute levels — you are always adding or subtracting gains and losses to a starting power level.

Where this shows up in practice

On a LoRa mesh, these units appear in a few specific places:

Transmit power. Typical community MeshCore nodes transmit between +14 and +22 dBm depending on hardware and regional configuration. The exact value is set in firmware and may be limited by regional regulations; most common boards operate well within regulatory limits.

Receiver sensitivity. LoRa radios can decode signals at deeply negative levels — approximately −100 to −137 dBm depending on the spreading factor, bandwidth, and coding rate (source: LoRa radio datasheets and modulation specification). This sensitivity is why LoRa achieves such long range even at low transmit power.

SNR. The SNR value you see in Live Packets and Network Stats is measured in plain dB — it is the ratio between the received signal level (in dBm) and the noise floor (also in dBm). A signal at −85 dBm arriving against a noise floor of −95 dBm shows an SNR of +10 dB. Understanding dBm is what lets you interpret why that +10 dB matters — and what it means when the noise floor rises or the signal weakens. The full treatment of SNR as a practical diagnostic is in Reading the Signal.

Antenna gain. Antenna gain is typically quoted in dBi (decibels relative to an isotropic radiator) or dBd (relative to a dipole). These are relative gains — dB values — which you add to or subtract from the transmit power (dBm) and cable losses (dB) to compute effective radiated power.

The moment these pieces connect — transmit power, cable loss, antenna gain, path loss, receiver sensitivity — you have a link budget. dBm is the unit that holds it all together. Working through a practical example — how much margin does a repeater placement decision actually require? — makes the arithmetic concrete.