dBm

decibel (dB) per milliwatt (mW)

The decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such as signal-to-noise ratio. The dBm is also dimensionless, but since it compares to a fixed reference value, the dBm rating is an absolute one.

The dBm is not a part of the International System of Units (SI) and therefore is discouraged from use in documents or systems that adhere to SI units (the corresponding SI unit is the watt).

However, the unit decibel (dB), without the 'm' suffix, is permitted for relative quantities, but not accepted for use directly alongside SI units. Ten decibel-milliwatts may be written 10 dB (1 mW) in SI.