dBm
Decibel-Milliwatts
The unit that quantifies signal power — for Wi-Fi, fiber light, and cellular alike. Once you understand its logarithmic, negative scale, the numbers in every signal-strength tool suddenly make sense.
dBm (decibel-milliwatts) measures signal power relative to a fixed reference of one milliwatt. It is the standard unit for expressing how strong a signal is across networking: Wi-Fi RSSI, fiber optic light levels, and cellular reception are all reported in dBm. Understanding it once unlocks the readouts in every Wi-Fi analyser, modem signal page, and phone field-test screen.
The reference point and the sign
The scale is anchored at the milliwatt: 0 dBm = 1 mW. From there:
- Positive dBm = more than 1 mW (strong transmitters, e.g. a router's output).
- 0 dBm = exactly 1 mW.
- Negative dBm = less than 1 mW (almost all received signals).
Since a Wi-Fi signal has lost most of its power by the time it reaches your device, received values are negative — and closer to zero is stronger. A received signal of -45 dBm is much stronger than -75 dBm.
Why it is logarithmic — the 3 and 10 rule
The "d" in dBm is the decibel, a logarithmic ratio, and this is what makes the unit powerful. Two shortcuts cover almost everything:
| Change | Effect on actual power |
|---|---|
| +3 dB | Roughly doubles the power |
| -3 dB | Roughly halves the power |
| +10 dB | Ten times the power |
| -10 dB | One tenth of the power |
So the gap between -60 and -70 dBm is not "a little weaker" — it is roughly a tenfold drop in power. This is why a few decibels can be the difference between a flawless connection and a flaky one.
Where you will meet dBm
- Wi-Fi: RSSI in dBm tells you coverage quality; aim for -67 dBm or better on important devices.
- Fiber: optical power (Rx/Tx light levels) is in dBm; a level far from the expected range signals a dirty connector or failing link.
- Cellular: phone signal in dBm (often labelled RSRP on 4G/5G) is more honest than the bars.
- Cable: while cable modems use dBmV rather than dBm, the same logarithmic logic applies to reading signal levels.
dBm vs the signal bars
The bars on your phone or laptop are a coarse translation of the underlying dBm value into three to five steps, and manufacturers map them differently — one phone's "three bars" may be another's "two." Reading the raw dBm removes that ambiguity and lets you compare locations precisely: walk around watching the number rather than the bars to find exactly where signal drops off. Paired with the noise level, dBm also feeds the signal-to-noise ratio, which is the figure that best predicts real-world speed.