Contents

**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:

- Convert dBm to Watt
- Add Watt values
- Convert the final result in Watt back to dBm

**Example Calculations**

- The sum of power levels 30 dBm and 3 dBm is
**30.01 dBm**(not 33 dBm). - The sum of three powers 40 dBm, 30 dBm and 10 dBm is
**40.42 dBm**(not 80 dBm).

**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*Log_{10}(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.

**Related Calculators and Posts**

- Subtract or Add dBm power
- dB to dBm conversion – when does it make sense?
- dBm Average