dBm Addition Calculator

Introduction

dBm stands for decibels (dB) relative to one milliwatt. It is an absolute unit of power. In this post we provide a calculator and explanation of how to add dBm. Additionally we give practical examples of dBm addition.

Calculator

This tool adds any number of dBm power values to compute the total power.

Simply enter the powers in dBm with each number separated by a Comma.


	

			

		

						

				

				

                

			

			
						

		

		






Explanation of How to Add dBm 





Since dBm is a logarithmic value, dBm values cannot be added directly. 





Instead we have to: 









Example Calculations





    

  • The sum of power levels 30 dBm and 3 dBm is 30.01 dBm (not 33 dBm).
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  • The sum of three powers 40 dBm, 30 dBm and 10 dBm is 40.42 dBm (not 80 dBm).
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dBm vs dB Addition Example





In this example using RF amplifiers and combiners, we show how dBm and dB addition rules work.





????  A detailed post on dB vs dBm





Unlike adding dB numbers, dBm quantities cannot be added. The important difference between the two is that dB represents a ratio while dBm is an absolute number (hence the m after dB).





Let’s get into some practical calculations – the kind that RF engineers perform regularly ????????





An amplifier provides an output that is 100 times the input. As a result, the gain in dB is given by 10*Log10(100) = 20 dB. Use the linear to dB calculator.










If we connect three of these in series, the total gain is the sum of the dB value of each: 20 + 20 + 20 = 60 dB. In this case, the output signal from the cascade is 1,000,000 times the input. Use the dB to linear calculator.










If the input to the amplifier is -50 dBm (or 10 nW using the dBm to Watt Calculator), the output of the first amplifier is -50 dBm + 20 dB = -30 dBm. The input signal is amplified by a factor of 100 to give an output of 1000 nW or 1mW which is the same as -30 dBm (using the Watt to dBm calculator). Alternatively we can also say that the output is 100 times the input (use the dB to times calculator).





Next let’s use an RF combiner to add signals prior to amplification. 










The inputs to the combiner are -40 dBm and -45 dBm.  Assuming no path loss, the dBm addition calculator on this page gives a combined power of -38.8 dBm. (Adding them together directly would give -85 dBm which is incorrect).





With 60 dB of total amplification, the output power after three amplifiers is -38.8 dBm + 60 dB = 21.2 dBm.





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