What does it mean to convert from dB to dBm?
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.
Similarly a dB value can be added or subtracted from a dBm value to get a final result in dBm.
What does this conversion mean? Let’s dive into the details.
👉 Here’s a detailed post on the differences between dB and dBm
Converter
Formula
dB = dBm1 – dBm2
dBm1 = dBm2 + dB
dBm2 = dBm1 – dB
Example Calculation and Explanations
If
- dB = 40 dB
- dBm2 = 30 dBm
then,
dBm1 = 40+30 = 70 dB
Let’s say a signal has a power level dBm1 and another signal has a level dBm2.
dBm1 – dBm2 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. But it’s not the difference – instead it’s the ratio of the two power levels.
The logarithmic (or dB) value of the ratio is then either added to or subtracted from a power level in dBm to get the other value.
*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 dB = dBm1 – dBm2 mean physically?
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.
Video explanation
This video explains dBm and dB using a spectrum analyzer