dBm = how loud the signal is. It's the absolute strength (0 dBm = 1 mW).
WiFi at the receiver is always negative — the closer to zero, the stronger:
- −30 dBm — excellent (right next to the router)
- −67 dBm — good
- −80 dBm — weak
- −90 dBm — almost nothing
dB = how much it changed (a difference, not a strength). Used for gains
and losses: “a wall eats 15 dB”, “an antenna adds 3 dB”.
The scale is logarithmic (in times, not plain +/−):
- −3 dB = 2× weaker (half the power)
- −10 dB = 10× weaker
- −20 dB = 100× weaker
- +10 dB = 10× stronger
So −60 dBm isn't “a bit worse” than −30 dBm — it's 1000× weaker.
How they work together (just add and subtract):
Received (dBm) = Emitted (dBm) − path loss (dB) − walls (dB)
e.g. 20 dBm − 73 dB (distance) − 16 dB (1 wall) = −69 dBm