How to convert from Watt to dBm

Why convert from Watt to dBm? dBm is convenient to work with. Most RF calculations such as Antenna Range, Two way radio and more are performed using dBm power levels. dBm is also used in conjunction with dB to compute input and output power levels. This involves simple addition or subtraction operations.

What is dBm?

dBm is a unit of power expressed in deciBels referenced to one milliwatt. It is commonly used in Radio frequency, Microwave and Optical communication systems as a measure of absolute power. It is used due to the ease of representing both very large and very small values with few digits.

For example 10,000,000,000,000 mW = 10¹³ mW = 130 dBm

As well, it makes RF budget calculations simpler than with linear values.

Watt to dBm Formula

P_dBm = 10*log₁₀(P_mW)

where

• P_mW is the power expressed in milliwatt.
• P_dBm is the power expressed in dBm

How to convert from Watt to dBm

To find the dBm equivalent of 1 W, first convert it to milliwatt. In this case, it is 1000. In general the value in Watt is multiplied by one thousand to convert to milliWatt.

The next step is to perform the calculation: 10*Log₁₀(1000)= 10*3 = 30 dBm.

1 Watt is therefore equal to 30 dBm. This equivalence is often used by RF engineers and a baseline from which other calculations can be performed.

Let’s take a look at an example below.

What is the dBm equivalent of 2 Watt? The same calculation can be performed to give 10*Log₁₀(2000) = 10*3.3 = 33 dBm.

Alternatively, we can see that 2 Watt is equivalent to two times 1 Watt or it represents a ratio of 2:1. This is linear ratio is equivalent to 3 dB. Therefore we can add 30 dBm + 3 dB to give 33 dBm.

With practice is becomes easier to identify these relationships for quick mental conversions but until then you can use the calculator on this page.

Why convert from Watt to dBm?

Many times a manufacturer will specify their product in terms of Watt. Retevis NR30 10w two way radio below is one such example.

However RF engineers will convert this to dBm. Here are a couple of reasons why:

dBm makes it easy to express large numbers. Numbers with many zeros as shown below can be expressed with a smaller number of dBm. For example:

• 0.0000000000001 Watt = -100 dBm
• 1.0000E-20 Watt = -170 dBm
• 10000000 Watt = 100 dBm

The Logarithmic scale makes this possible.

dBm is convenient to work with. Most RF calculations such as Antenna Range, Two way radio and more are performed using dBm power levels. dBm is also used in conjunction with dB to compute input and output power levels. This involves simple addition or subtraction operations.

Watt to dBm Table

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