How to convert from dB (deciBels) to dBm (deciBel-milliwatts)?
dB is relative number while dBm is an absolute number. There is no way to convert dB to dBm.
You can however subtract two dBm numbers and the result is a dB value.
dB = dBm1 – dBm2
- dBm1 = 40 dBm
- dBm2 = 30 dBm
dBm1 – dBm2 = 10 dB
Note that this is not the difference in power level between the two. For that use this calculator.
Instead it 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. For instance, this calculator lets you convert between Volt and dBm.
dB 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.
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