Occasionally one can see texts and reports about RF circuits where people mix up the terms dB and dBm.

The typical erroneous statement is something like “The power level is 7 dBm lower”. In most, probably all, cases it should instead say “The power level is 7 dB lower.”

Let’s explain by example:

Say we have a power level of 20 dBm. This means 20 dB above 1 mW, i.e. 100 mW.

And 7 dB is a factor of 5, so 7 dBm is 5 mW.

Now 20 dBm – 7 dB = 13 dBm = 20 mW. **Subtracting a value in dB means a division.** I.e. 100 mW / 5 = 20 mW.

But **subtracting a value in dBm means taking the difference between two absolute levels**: 20 dBm – 7 dBm = 100 mW – 5 mW = 195 mW = 19.8 dBm.

Thus it very rarely makes sense to say “xx dBm lower / higher” or “the power was reduced / increased by xx dBm”. Use dB in these cases instead!