Free Space Path Loss Calculator

What Is Free-Space Path Loss?

Free-space path loss (FSPL) is the signal attenuation between two isotropic antennas in an obstacle-free environment. It grows with distance and frequency and is a baseline term in RF link budgets for Wi-Fi, cellular, satellite, and radar systems.

FSPL Formula

$$FSPL = 20\log_{10}(d) + 20\log_{10}(f) + 20\log_{10}\left(\frac{4\pi}{c}\right)$$

Distance $d$ is in meters, frequency $f$ in hertz, and $c \approx 2.998 \times 10^8$ m/s is the speed of light. The result is in decibels (dB). At 1 km and 2.4 GHz, FSPL is about 100 dB.

Link Budget Context

Received power equals transmit power minus FSPL plus antenna gains minus other losses. FSPL does not include absorption by buildings, rain, or foliage—those are additional margins in a real link budget.

Frequently Asked Questions

Why does FSPL increase with frequency?

Higher frequency means shorter wavelength. For the same antenna aperture, shorter wavelengths spread energy over a wider solid angle, so received power drops.

What distance and frequency units does the formula use?

The standard FSPL equation uses meters for distance and hertz for frequency. This calculator accepts common units and converts internally.

Does FSPL include antenna gain?

No. FSPL assumes isotropic antennas. Add transmit and receive antenna gains separately in your link budget.

What is a typical FSPL for Wi-Fi?

At 2.4 GHz over 100 m, FSPL is about 80 dB. At 5 GHz over the same distance it is about 86 dB because of the higher frequency term.

How do I convert FSPL dB to a power ratio?

Power ratio = 10^(−FSPL/10). For example, 100 dB FSPL means received power is 10⁻¹⁰ of transmitted power in free space.

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Free Space Path Loss Calculator

What Is Free-Space Path Loss?

FSPL Formula

Link Budget Context

Frequently Asked Questions

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