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Decibel-Milliwatt (dBm) Calculator

dBm states power in decibels against a 1 milliwatt reference. Enter the power in any one box - milliwatts, watts, dBm or dBW - and the other three units appear together.

How to convert to dBm

dBm is an absolute power unit on a logarithmic scale, referenced to 1 milliwatt: P(dBm) = 10 × log10(P(mW) / 1 mW). By that definition 1 mW = 0 dBm, and 1 W = 1000 mW = 30 dBm.

From milliwatts, take 10 times the base-10 logarithm of the number of milliwatts: 100 mW gives 10 × log10(100) = 20 dBm. From watts, scale by 1000 first (equivalently add 30 dB): 100 W = 100000 mW gives 50 dBm.

How to convert from dBm

Invert the definition: P(mW) = 1 mW × 10^(P(dBm) / 10), so 20 dBm gives 10^2 = 100 mW. For watts subtract 30 first: P(W) = 1 W × 10^((P(dBm) - 30) / 10), so 40 dBm gives 10^1 = 10 W.

dBW is the same ladder referred to 1 watt, so the two scales sit exactly 30 apart: 40 dBm = 10 dBW, and 20 dBW = 50 dBm.

dB is not dBm

A bare dB is a relative ratio used for gain, with no fixed reference, so it cannot be converted to dBm on its own. Start from a known power: a 3 dB gain doubles 10 mW to 20 mW, i.e. from 10 dBm to just over 13 dBm.