# Why are dBm values negative?

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.