How to convert from dB (deciBels) to dBm (deciBel-milliwatts)?
dB is relative number while dBm is an absolute number. There is no explicit way to convert dB to dBm. Two dBm numbers can however be subtracted from one another and the result is a dB value.
What does this conversion mean? Let’s dive into the details.
dB = dBm1 – dBm2
Example Calculation and Explanations
- dBm1 = 40 dBm
- dBm2 = 30 dBm
dBm1 – dBm2 = 40-30 = 10 dB
Note that this is not the difference in power level between the two*. That’s a common mistake in the RF lab when using a spectrum analyzer. Users will put markers on two peaks in the spectrum. The instrument will compute the difference in dB.
*Use this calculator to find the sum or difference between two power levels expressed in dBm. Using it we can see that the difference between 40 dBm and 30 dBm power levels is 39.54 dBm.
As an example, the output shows the use of delta markers in an RF Explorer handheld Spectrum Analyzer.
Note the difference between M1 and M2 is 33 dB which is the same value using the calculator on this page with inputs -29.5 dBm and -62.5 dBm.
What does this number mean?
This number 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 representing power. For instance, these calculator lets you convert from
dB on the other hand, 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.
This video explains dBm and dB using a spectrum analyzer