Free Space Path Loss Calculator converts distance, frequency, and antenna gains into isotropic and net path loss for RF engineers and wireless ISPs planning link budgets today.
Calculate Free Space Path Loss for RF Link Budgets and Range Planning
The Free Space Path Loss Calculator converts distance, frequency, and antenna gains into the signal loss a link must overcome between two points with a clear line of sight. RF engineers, wireless ISPs, and satellite and microwave link planners use it to size link budgets before committing to hardware or tower placement.
How to Use
Enter transmission distance (d) in m, km, ft, or mi; operating frequency (f) in Hz, kHz, MHz, or GHz; and transmitter gain (Gt) and receiver gain (Gr) in dBi. The calculator returns isotropic path loss, net path loss after antenna gains, and first Fresnel zone clearance.
Formula
Per ITU-R Recommendation P.525, the free-space basic transmission loss between two isotropic antennas is:
$$L_{bf(dB)} = 20\log_{10}(d_{km}) + 20\log_{10}(f_{GHz}) + 92.45$$
Antenna gains then offset that isotropic figure to give the net path loss the link budget actually has to close:
$$PL_{net(dB)} = L_{bf(dB)} – G_{t(dBi)} – G_{r(dBi)}$$
Worked example: at $d = 10$ km and $f = 2.4$ GHz, $L_{bf} = 20\log_{10}(10) + 20\log_{10}(2.4) + 92.45 = 120.05$ dB. With a 10 dBi antenna at each end, $PL_{net} = 120.05 – 10 – 10 = 100.05$ dB. The most common input mistake is mismatching units against the constant — entering distance in miles or frequency in MHz while the 92.45 dB constant assumes kilometers and gigahertz throws the result off by tens of decibels.
Diagram
Reference Table
| Constant / Criterion | Value | Source |
|---|---|---|
| Free-space loss constant (d in km, f in GHz) | 92.45 dB | ITU-R P.525 |
| Path loss increase per decade of distance or frequency | 20 dB | ITU-R P.525 |
| Minimum recommended Fresnel zone clearance for near free-space propagation | 60% of first Fresnel zone radius | ITU-R P.530 |
FAQ
What does free space path loss actually measure?
It’s the signal loss caused purely by the wavefront spreading out over distance in an unobstructed vacuum or atmosphere, with no antenna gain, terrain, or weather included. It’s the theoretical floor every real link has to beat.
Does FSPL account for rain fade, foliage, or obstructions?
No. FSPL is the free-space case only. Real link budgets add separate margins for rain attenuation, atmospheric absorption, foliage, and diffraction loss on top of the FSPL figure, per the relevant ITU-R propagation recommendations for each effect.
Why does the calculator show two different loss values?
Isotropic path loss is the raw loss between two 0 dBi antennas. Net path loss subtracts your actual transmitter and receiver gain from that figure. Mixing the two up — using isotropic loss as if it already includes antenna gain — understates the real link margin.
Why does a lower-frequency link need more Fresnel zone clearance despite lower path loss?
Fresnel zone radius scales with the square root of wavelength, so lower frequencies have longer wavelengths and wider zones. A link with less path loss can still demand more physical clearance around obstacles than a higher-frequency link on the same route.
Is a bigger antenna gain always better for closing the link budget?
Gain reduces net path loss dB-for-dB, but very high-gain antennas narrow the beamwidth, making alignment more sensitive to mounting drift and wind sway. Past a certain point, pointing accuracy limits the link more than raw path loss does.