“How To Change from Linear Power to dB Power and Vice-Versa?”
dBm (sometimes dBmW or decibel-milliwatts) is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). Since it is referenced to the watt, it is an absolute unit, used when measuring absolute power.
By comparison, the decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such a signal-to-noise ratio.
As well known, Decibels express a power ratio and not an amount. Decibels are logarithmic, not linear. In fact, 20 dB is not twice the power ratio of 10 dB but 13dB is twice of 10 dB.
The defining equation for decibels is:
where P2 is the power being measured, and P1 is the reference to which P2 is being compared.
To convert from decibel measure back to power ratio:
dBm is defined as the dB using a reference P1 equal to 1 mW (Radio frequency, 50 Ohm)