Subtract dBm Calculator

dBm stands for decibels (dB) relative to one milliwatt. It is an absolute unit of power.

How you subtract two dBm values together to find the total power?

This tool performs the subtraction of two dBm power values. Simply enter Power1 and Power2 with the appropriate units. It computes the difference

Power1 – Power2

in dBm or dBW.


Note that since dBm is a logarithmic value you cannot subtract two dBm values directly. Instead you have to:

  • Convert dBm to Watt
  • Subtract Watt values
  • Convert the result back to dBm

Example calculation

The difference between power levels 30 dBm and 3 dBm is 29.99 dBm (not 27 dBm).

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