dB to dBm Calculator

How to convert from dB (deciBels) to dBm (deciBel-milliwatts)?

dB is relative number while dBm is an absolute number. There is no way to convert dB to dBm.

You can however subtract two dBm numbers and the result is a dB value.


dB = dBm1 – dBm2



  • dBm1 = 40 dBm
  • dBm2 = 30 dBm


dBm1 – dBm2 = 10 dB

Note that this is not the difference in power level between the two. For that use this calculator.

Instead it is the ratio of powers which has no units.

  • dBm1 = 40 dBm = 10 Watt
  • dBm2 = 30 dBm = 1 Watt

The ratio between the two is 10 and on the log scale it works out to:

10*Log10(10) = 10 dB

Why can you not convert between dBm and dB?

dBm is an absolute number. For instance, this calculator lets you convert between Volt and dBm.

dB is a relative number and represents a ratio between two quantities. In communications engineering, dB is used to represent the signal-to-noise ratio, gain of an amplifier and more.

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