Free-Space Path Loss Calculator
Enter a path distance and frequency to get the ideal line-of-sight loss in decibels — the starting point for any RF link budget.
Where path loss fits
Path loss is the single biggest number in most links, and it is set by two things you often cannot change: how far apart the radios are and what frequency they use. Everything else in the design — antenna gain, transmit power, receiver sensitivity — exists to overcome it. Getting the free-space figure right first tells you how much margin the rest of the system has to find.
Feed this loss straight into the RF Link Budget Calculator, size the antennas that recover it with the Antenna Gain Converter, and set element lengths for the band with the Frequency & Wavelength Calculator.
Frequently Asked Questions
What is free-space path loss?
Free-space path loss (FSPL) is the attenuation a radio signal suffers as its energy spreads out over an unobstructed line-of-sight path — no terrain, buildings, foliage, or reflections, just the geometric spreading of the wavefront and the frequency-dependent effective aperture. It is the baseline every link budget starts from, and it is always optimistic: a real path only loses more.
Which formula does this use?
The decibel form of the Friis equation: FSPL(dB) = 20·log10(d) + 20·log10(f) + K, where K folds in the units. This calculator uses distance in kilometres and frequency in megahertz, giving K = 32.44. Enter miles, metres, or gigahertz and it converts to km and MHz first, so the constant stays consistent.
Why does loss grow with frequency if higher bands sound 'better'?
For a fixed antenna gain, a higher frequency has a smaller effective aperture, so FSPL rises 6 dB every time the frequency doubles — just as it rises 6 dB every time the distance doubles. Higher bands look worse on paper here; in practice engineers claw the loss back with higher-gain (more directional) antennas, which is exactly why millimetre-wave links use tight beams.
Can I trust this number for a real deployment?
Treat it as the best case. FSPL ignores obstruction, diffraction, atmospheric and rain absorption, ground reflection, and multipath fading, all of which add loss on a real path. Use the figure as the path-loss input to a full link budget with a fade margin, and validate against measurement before you commit hardware. It is an educational estimate, not a propagation guarantee.
Ideal line-of-sight estimate only. Real paths add terrain, foliage, rain, and multipath losses. Not engineering or regulatory advice.