It is very common to see signal strength expressed as a negative number with the units “dBm,” which stands for decibel-milliwatts. But what do these numbers mean, and how should you compare them? And why are they negative?
On the dBm scale, a value of zero indicates 1 milliwatt of power. The decibel is a logarithmic scale, so every 3dBm increase represents a doubling of power, and every 10dBm represents a 10x increase. So 3dBm represents 2mW (milliwatts), and 10dBm represents 10mW, and and 20dBm represents 100mW, and so on.
The pattern works for negative numbers too. −3dBm is half a milliwatt, and −10dBm is 1/10th of a milliwatt.
By the time a radio signal reaches a receiver, the amount of power left is incredibly small – and thus is measured in negative dBm. A great signal for a wireless device would be a −50dBm signal strength. A barely useable signal is −100dBm, which is 100,000 times weaker. And even the most sensitive receivers will have a hard time picking up a −110dBm cellular signal, which is a million times weaker than a −50dBm signal.
It is amazing that such sensitive technology exists and can be packed into your pocket. But GPS radios are even more amazing – the typical signal received from a GPS satellite is around −127dBm!« Back to Glossary Index