Watt to dBm Calculator

This tool converts Watt to dBm.

Enter the value and select either Watt, Milliwatt, Microwatt, Nanowatt or Picowatt as units.



???? dBm to Watt Calculator

Watt to dBm Formula

PdBm = 10*log10(PmW)


  • PmW is the power expressed in milliwatt.
  • PdBm is the power expressed in dBm

What is dBm?

dBm is a unit of power expressed in deciBels referenced to one milliwatt [1]. 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.

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. The power amplifier below is one such example.

BTECH AMP-U25 Amplifier for UHF (400-480MHz), 20-40W Output (2-6W Input), Analog and Digital Modes, Compatible with All Handheld Radios: BTECH, BaoFeng, Kenwood, Yaesu, ICOM, Motorola

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, Wi-Fi RSSI 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


Frequently Asked Questions

What is 0 Watt in dBm?

Zero watt equivalent to zero milliwatt and the Logarithm of 0 is a very tiny or infinitesimally small number. It is therefore indicated as -∞.

What is 0 dB in dBm?

It is not possible to convert dB to dBm and therefore this question is meaningless.

Related Calculators

  • dBm Average – use this tool to compute the average of any number of signals with power expressed in dBm
  • dBm vs dB – what’s the difference?


[1] dBm article on Wikipedia