V/m to dBm and Watt Calculator

This calculator converts Field Intensity Volt-Per-Meter (V/m) to Power in dBm and Watt.

V/m is a measure of field intensity at any distance from the transmitter or receiver.

The signal at the input of the receiver however is typically specified in terms of Volt or dBm or Watt.

There are two calculators to convert from V/m to dBm and Watt.

The first one requires

  • Antenna gain and
  • Frequency of operation

It uses this to compute the Antenna Factor, RMS voltage and then Power in dBm.



???? dBm to V/m (for a transmitter)


AF = 20*Log10(9.73/(λ√G))

V = E/10(AF/20)

VdBμV = EdBμV/m – AFdB/m

PdBm = VdBμV − 10*Log10(Z) − 90

PWatt = 10((PdBm – 30)/10)


  • AF = Antenna Factor (dB/m)
  • λ = Wavelength
  • G = Antenna Gain
  • PdBm = Power in dBm
  • PWatt = Power in Watt

Convert Antenna Factor to Power

This next calculator is used if you know the antenna factor. Many antenna vendors will provide this in a data sheet. The unit of AF is dB/m.

Typically and practically, the Antenna factor varies with frequency. Use the value of AF at a specific frequency to give you the power in dBm at that frequency as well.




V = E/10(AF/20)


Antenna vendors typically specify antenna factor by way of a plot as shown in the picture below. This is the antenna factor for a Horn made by Com-power.

The field intensity is specified at 1m, 3m and 10m. These distances are important from an FCC standpoint.

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