dB vs dBm – What’s the difference?

dB is often confused with dBm.

They are very different.

dB stands for deciBel and dBm stands for deciBel relative to 1 milliwatt. Let’s get into the details.

What is dB?

dB is a relative measure of two power or amplitude levels

It can be used in the following instances :

  • Gain of an amplifier – the ratio of the amplifier output to input power
  • Link margin – specifies the ratio between the received signal and the minimum detectable signal power
  • Signal-to-noise ratio – often abbreviated as SNR, this gives a measure of how much the signal power exceeds the noise
  • Antenna cable loss – what is the insertion loss of the RF cable that sits between the antenna and transmitter or receiver?

The equation for calculating amplifier gain in dB is

Amplifier GaindB = 10*Log10(Pout/Pin)

Similarly, the Signal-to-noise ratio in dB is

SNRdB = 10*Log10(Psignal/Pnoise)

The dB scale presents a convenient method of representing large numbers. For instance 1000000000 = 90 dB. It is used in audio, radio and other engineering disciplines.

What is dBm?

dBm is a measure of power level and is an absolute quantity

The input or output power level of an RF circuit can be expressed in dBm.

Power in Watt (PW) is calculated from dBm (PdBm) using the following formula:


Example applications:

  • Sensitivity of a Wi-Fi system – What is the lowest signal level at which your home or office wireless system is functional?
  • Thermal noise power – used to determine the sensitivity of a radio receiver.

dBm is normally used in Radio Frequency and Optical Engineering Applications.

Can dB be converted to dBm?

No – dB cannot be converted to dBm. They are different from each other.

However, the difference between two dBm quantities is a number in dB.

For instance, the noise power in dBm can be subtracted from the signal power in dBm to give the SNR in dB.

SNRdB = PsdBm – PndBm

If the signal power is +10 dBm and the noise power is -50 dBm, the SNR is 60 dB.

SNRdB represents the ratio between the two linear quantities of signal and noise power. Another example is the ratio between EIRP and ERP and the difference between the two in dBm.