Signal is not a bar count

A number, not bars

dBm is power compared with one milliwatt: 0 dBm is 1 mW, +20 dBm is 100 mW, −70 dBm is a ten-thousandth of a microwatt. Every 3 dB is about double or half the power, every 10 dB ten times. A Wi-Fi receiver works with powers this small, which is why the scale is logarithmic.

The board gives you the number. WiFi.RSSI() in Arduino and the rssi field of a scan record in ESP-IDF are in dBm, and they are the chip's own estimate, not a calibrated measurement. Bars are your phone's own summary; no standard says how many bars a reading earns, and a summary can hide a drop to a quarter of the power.

Next to a router you may see the −30s. Below about −80, links often get unreliable. That is a rule of thumb, not a rule: the weakest usable signal depends on the data rate, which is lesson 9.

Distance, walls, echoes

In open space, doubling the distance costs about 6 dB, a quarter of the power, and at 2.4 GHz the first metre already costs about 40 dB. A building breaks that simple rule. A wall weakens the signal by an amount that depends on what it is made of, how thick it is and the frequency; metal and reinforced concrete cost most, and a wall rarely stops it completely. Waves also bend round edges, so the space behind a wall's end is not dark.

Reflections arrive by several paths and add or cancel. Moving a few centimetres can change the reading a lot: one spot can be poor and the spot a hand's width away fine.

With the same antennas, 2.4 GHz often reaches further indoors than 5 GHz. Often, not always.

Strong is not fast

The signal says how well the phone hears the access point. It says nothing about who else is on the channel. Stations get roughly equal chances to send, so with three busy neighbours the phone gets about a quarter of the turns, at full bars. Sharing the air is lesson 7; when a weak network is one of several, WiFiMulti picks the strongest it can hear.

Common mistakes

• Reading bars as speed. Bars summarise the signal, the phone's own way. A busy channel is slow at full bars.
• Treating −80 or any other figure as a cliff. The weakest usable signal depends on the rate, and the thresholds people quote are rules of thumb.
• Moving the router before moving the board. Echoes make spots a hand's width apart read very differently; try a few centimetres first.