Bluetooth Signal Strength Explained

What the number is

Bluetooth reports received signal strength, usually in dBm, and it is always negative. Closer to zero means stronger. Around −30 dBm is very close, roughly touching distance. −60 is the same room. −80 and below is far, or through a wall.

It is a measure of received power, not of distance. Distance is inferred from it, and the inference is rough.

Why it jumps around

Radio at 2.4 GHz is absorbed by water, and a human body is mostly water. Turning around can change the reading by 10 dBm or more.

Metal reflects. A device behind a radiator or inside a metal cabinet can read stronger in a doorway than beside it.

Transmit power varies by device. A speaker and a coin-cell tag at the same distance produce very different numbers, so never compare two different devices to each other, only a single device against itself over time.

How to use it properly

Read the trend, not the value. Three seconds in one position, then move, then three more.

Sweep in straight lines and turn 90 degrees at each peak. Two passes usually put you within a metre.

Hold the phone away from your body and low. Your own torso is one of the biggest sources of attenuation in the room.

A strong signal with nothing visible means the device is inside or underneath something. Search the volume, not the surface.

Common questions

Can signal strength tell me exact distance?

No, and an app that shows you a precise metre figure from signal strength alone is presenting a guess as a measurement. What is reliable is the direction of change.

What is a good RSSI value?

There is no universal good value because transmit power varies by device. Judge a device against its own earlier readings, not against a fixed scale.

In the app: The scan shows every nearby device with its name, battery and live signal strength. Read about Proximity Radar.

Reading Bluetooth signal strength (RSSI)