RF · propagation
Free space path loss calculator.
Loss between two isotropic antennas in free space, from the ITU-R P.525 equation.
Inputs
Free space loss
Enter values to calculate.
How do I calculate free space path loss?
ITU-R P.525 gives the free-space basic transmission loss between two isotropic antennas as Lbf = 20 log10 (4 pi d / lambda) dB, with distance d and wavelength lambda in the same unit. In the common form with frequency in MHz and distance in km this becomes 32.4 + 20 log10 f + 20 log10 d. This tool uses the exact first form, so it differs from the rounded 32.4 constant by a few hundredths of a dB.
Worked example. At 2.4 GHz over 1 km the loss is 100.1 dB. At 900 MHz over 10 km it is 111.5 dB. Doubling the distance or the frequency always adds 6.02 dB.
Free space means a clear line of sight with no ground reflection, walls, foliage or rain. Real links have extra loss, so treat this as a best case and use the link budget calculator to add margin. The formula is for the far field, so the tool asks for at least one wavelength of distance.
Frequently asked questions
What is the free space path loss formula?
Lbf = 20 log10 (4 pi d / lambda) dB, or 32.4 + 20 log10 f(MHz) + 20 log10 d(km), from ITU-R P.525.
Why does higher frequency lose more?
The formula depends on frequency because an antenna of fixed gain captures less power at shorter wavelengths. Doubling the frequency adds 6 dB.
Is FSPL the same as real-world loss?
No. It is the ideal minimum. Obstructions, ground effects and multipath make real loss higher.
How much is the loss at 2.4 GHz and 1 km?
About 100.1 dB.
Sources
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The formula is from the ITU-R text. No other figures are used. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Ideal free-space estimate. It ignores obstructions, reflections, atmosphere and rain, so real links lose more.