Free Space Path Loss Calculator
Calculate free-space path loss for any distance and frequency, the theoretical floor that every wireless link budget starts from.
Result
Same distance, other bands
| Frequency | FSPL at 5 km | vs your frequency |
|---|---|---|
| 2400 MHz | 114.02 dB | -7.2 dB |
| 5500 MHz | 121.23 dB | +0 dB |
| 5800 MHz | 121.69 dB | +0.46 dB |
| 6000 MHz | 121.98 dB | +0.76 dB |
| 11000 MHz | 127.25 dB | +6.02 dB |
| 24000 MHz | 134.02 dB | +12.8 dB |
| 60000 MHz | 141.98 dB | +20.76 dB |
The maths
- Formula
- FSPL(dB) = 20·log10(d_km) + 20·log10(f_MHz) + 32.44
- Doubling distance
- +6 dB loss
- Doubling frequency
- +6 dB loss
About Free Space Path Loss Calculator
Free space path loss is how much signal a radio wave loses simply by spreading out over distance. It sets the floor for every link budget: no antenna alignment or better radio recovers loss that the geometry already imposed.
Beyond free space
FSPL assumes an unobstructed path with no reflections, which almost never holds. Indoors, drywall costs 3 to 5 dB per wall and concrete 10 to 15 dB; foliage costs several dB per metre of penetration and worsens when wet. Over water, reflections create multipath fading that varies with tide and temperature. Empirical models (Hata, COST-231, ITU indoor) exist precisely because free space is the floor rather than the answer — treat FSPL as the best possible case and add measured or modelled loss on top.
Why higher frequencies lose more
The loss term contains frequency because a fixed-size antenna captures a smaller fraction of the wavefront as wavelength shrinks. Moving from 2.4 GHz to 5.5 GHz costs about 7 dB at the same distance - part of why 5 GHz coverage indoors is shorter, even before wall attenuation. It also explains why millimetre-wave links are measured in hundreds of metres, not kilometres.
Common use cases
- Sanity-checking whether a proposed point-to-point link is remotely plausible.
- Comparing band options for the same hop distance.
- Explaining why a link that worked at 2.4 GHz struggles at 5 GHz.
Edge cases and gotchas
- FSPL ignores obstruction, rain fade and multipath entirely - it is a starting point, not a design.
- Rain fade becomes significant above about 10 GHz and dominates above 24 GHz.