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Free Space Path Loss Calculator

Enter the distance in km and frequency in MHz to get the free-space path loss in dB.

How it is calculated

The tool uses FSPL (dB) = 20·log10(d) + 20·log10(f) + 32.44, with d in kilometres and f in megahertz. The constant 32.44 only holds for exactly those units. The result is a loss in decibels with two decimals, and both inputs must be above zero. It models an unobstructed line of sight in open space: it includes no walls, terrain, rain, antenna gain, cable loss or reflections, so real paths lose more. Because of the logarithms, doubling either the distance or the frequency adds about 6 dB, and a tenfold increase adds 20 dB. If the formula would give a negative loss, which happens only at extremely short distances and low frequencies right next to the antenna, the tool shows no result because the far-field formula does not apply there.

Worked example

A 5.8 GHz point-to-point link covers 5 km. Convert the frequency to megahertz: 5800 MHz. Then 20·log10(5) ≈ 13.98 and 20·log10(5800) ≈ 75.27. Add the constant: 13.98 + 75.27 + 32.44. The free-space path loss is 121.69 dB. Moving the same link to 2400 MHz would give 114.02 dB, about 7.7 dB less.

Try this example →

Common mistakes

  • Entering the frequency in GHz or Hz. The field is megahertz, so 5.8 GHz is 5800.
  • Entering the distance in metres or miles. The constant 32.44 requires kilometres, so 500 m is 0.5.
  • Treating the result as the total loss of a real link. Walls, trees, rain and Fresnel-zone blockage come on top of free-space loss.
  • Forgetting antenna gains and cable losses. Free-space loss is only one term of a link budget; transmit power, gains and receiver sensitivity are separate.
  • Using the formula at very short distances near the antenna. There it no longer applies, and the tool shows no result when the computed loss would be negative.

About this calculator

This calculator applies the standard Free-Space Path Loss (FSPL) formula: FSPL(dB) = 20·log₁₀(d) + 20·log₁₀(f) + 32.44, with distance d in kilometres and frequency f in megahertz. It's derived from the Friis transmission equation and describes how much a radio signal weakens purely from spreading out over distance in open space, with no obstructions, reflections or atmospheric absorption.

RF engineers, wireless ISPs and radio amateurs use FSPL as the baseline term in a link budget — the calculation that adds transmitter power and antenna gains, then subtracts path loss and cable losses, to check whether a receiver will get enough signal. Doubling the distance or the frequency each adds about 6 dB of loss.

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