# Add or Subtract dBm Power

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

How to add or subtract two power values (in dBm) to find the total combined power?

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

Power1 – Power2 or Power1 + Power2

in dBm or dBW.

## Explanation

Note that since dBm is a logarithmic value you cannot add or subtract two dBm values directly

## Example calculation

• The difference between power levels 30 dBm and 3 dBm is 29.99 dBm (not 27 dBm).
• The sum of power levels 30 dBm and 3 dBm is 30.01 dBm (not 33 dBm)

## RF Combiners

An RF combiner is used to combine or add two signals. The calculator can be used to find the sum of signals input to the two ports (1 and 2).

Use the calculator above to find the signal level (in dBm) at Port S. If +3 dBm and +4 dBm are input at Ports 1 and 2 respectively, the combined power is 6.54 dBm.

In practice there will be some insertion loss between Port 1 and Port S and Port 2 and Port S. This will reduce the combined power level.

For example this RF power combiner has an insertion loss that increases with frequency. The IL has a max value of 1.4 dB at the upper end of the frequency range.

Revisiting the example above, if +3 dBm and +4 dBm are injected at the two ports, the output can be as low as 6.54 – 1.4 – 1.4 = 3.74 dBm. Here we subtracted 1.4 dB for each channel of the combiner and the final result is in dBm.