Most measurements on phones or laptops for signal strengths of Wi-Fi, 4G or 5G networks are negative dBm values.
The reason for this is that the received signal power is less than one milliwatt. The dBm equivalent is negative for these power levels.
Detailed Explanation
➡️ Most devices report received power levels in dBm. dBm is a logarithmic unit that expresses the power level relative to 1 milliwatt (mW).
What is the relation between milliwatt and dBm?
PdBm=10*Log10(PmW)
where
- PdBm is the power in dBm
- PmW is the power in milliwatts
The calculator below gives the dBm equivalent of a power in mW.
A value of 1 mW is equivalent to 0 dBm
Any value greater than 1 mW will result in a positive dBm value. A value less than 1 milliwatt gives a negative dBm value.
- 10 milliwatt is equivalent to 10 dBm
- 0.1 milliwatt is equivalent to -10 dBm
Since most devices receive signals lower than 1 milliwatt, the reported power is a negative value in dBm units
What does a negative dBm reading mean?
A negative dBm reading means the received signal strength at the antenna of your wireless device (cellphone, laptop or tablet) is less than 1 milliwatt.
This is a weak signal when you consider that a milliwatt is one-thousandth of a watt.
However for a wireless device it is quite strong. A 0 dBm signal is very strong. In fact anything greater than -60 dBm is very good. Only a signal lower than -90 dBm is weak.