dBm to Distance Calculator (with Examples)

This tool uses the transmitter power and receiver sensitivity in dBm to calculate the maximum distance (miles or km) that can be achieved in a communication system.

The system includes a transmitter, receiver, transmit and receive antenna.

To calculate the max distance, enter the following numbers in the calculator:

  • Transmitter Power (dBm)
  • Receiver Sensitivity (dBm)
  • Frequency of operation (Hz/KHz/MHz/GHz)
  • Transmit Antenna Gain (dBi)
  • Receive Antenna Gain (dBi)
  • Total Cable and Other Losses (dB)

Formula

The calculator uses the Free Space Path Loss (FSPL) equation:

FSPL = 20*Log10(d) + 20*Log10(f) + Lc + 20*Log10(4π/c) – GTx – GRx

to calculate d

20*Log10(d) = FSPL – 20*Log10(f) – Lc – 20*Log10(4π/c) + GTx + GRx

d = 10^((1/20)*(FSPL – 20*Log10(f) – Lc – 20*Log10(4π/c) + GTx + GRx))

Example Range Calculation

What is the range of a 20 dBm Wi-Fi signal?

At a frequency of 2450 MHz (ISM band) and transmit power of +20 dBm, let’s estimate at the range of a Wi-Fi system in free space.

With a receive sensitivity of -90 dBm, transmit and receive antenna gain of 0 dBi, and 0 dB cable loss, the maximum distance is 3.1 km or 1.9 miles.

Important to remember that this calculation is outdoors and free space which is an ideal condition.

Assuming indoor conditions, using our indoor Wi-Fi range calculator the same level of received signal is achieved at only 55 meters. A much smaller range. This is to be expected as signals are attenuated more indoors than in free space.

A 20 dBm signal will have greater range outdoors than indoors.

How to use the Calculator

Here is an explanation of the variables in the equation.

Free Space Path Loss

Abbreviated as FSPL this is simply the difference between the transmit and receive power in dBm. The difference is expressed in dB (explained here).

Frequency of operation

This is the frequency at which the communication system operates. The range is higher at lower frequencies, so entering the highest operating frequency will give you an upper limit on the achievable range. In the case of L-band for instance, use f = 2.1 GHz.

Transmit Power

This is the power at the output connector of the transmitter. It is usually specified in dBm. If specified in Watt, use this calculator to convert to dBm.

Transmit Antenna Gain

This depends on the type of antenna used and is expressed in dBi. It can be calculated from the Antenna Factor and frequency of operation.

Receiver Sensitivity

Minimum level of input signal that a radio receiver can detect and demodulate. Use this calculator to find the sensitivity.

Receive Antenna Gain

Depends on antenna used and is expressed in dBi. In cases where the signal is from a particular direction, a high gain antenna (8 dBi for instance) can be used. This allows focusing of energy instead of from all directions. It can be calculated from the Antenna Factor and frequency of operation

Total Cable and Other Losses

This includes losses between connectors and antennas. If splitters are used, the loss should be accounted for as well. It is specified in dB. Use this calculator to find the loss due to antenna cable.