FSPL Calculator
Estimate free space path loss for RF, microwave, and wireless links.
What this free space path loss calculator does
This calculator computes free space path loss (FSPL), which is the signal attenuation that happens as a radio wave spreads out over distance in ideal free-space conditions. It is widely used in wireless engineering for Wi-Fi links, microwave backhaul, satellite communication, telemetry, radar planning, and general RF link budgets.
FSPL is expressed in decibels (dB). A higher FSPL value means more loss and a weaker received signal, all else being equal.
Core equation
The calculator uses the standard form below after converting your units:
Equivalent versions exist for other unit systems, such as:
- 92.45 + 20·log10(d[km]) + 20·log10(f[GHz])
- 32.44 + 20·log10(d[miles]) + 20·log10(f[MHz]) + 4.68 (converted form)
How to use the calculator
- Enter link distance and select the correct distance unit.
- Enter frequency and select its unit.
- Click Calculate Path Loss to get FSPL instantly.
- Optionally provide TX power, antenna gains, and extra losses to estimate received power.
Example
Suppose you have a 2.4 GHz link over 5 km:
- Distance = 5 km
- Frequency = 2.4 GHz = 2400 MHz
- FSPL ≈ 32.44 + 20·log10(5) + 20·log10(2400) ≈ 114.02 dB
If TX power is 20 dBm and each antenna has 8 dBi gain with 2 dB of miscellaneous losses, estimated received power is:
Pr = Pt + Gt + Gr − FSPL − L = 20 + 8 + 8 − 114.02 − 2 = −80.02 dBm
What affects free space path loss most?
1) Distance
Loss increases as distance increases. Doubling distance adds approximately 6 dB of loss.
2) Frequency
Higher frequencies experience higher FSPL for the same distance. Doubling frequency also adds about 6 dB of loss.
Important limitations
FSPL assumes ideal free space. Real deployments usually include additional impairments:
- Obstructions (buildings, trees, terrain)
- Multipath fading and reflections
- Atmospheric absorption and rain fade (especially at higher microwave bands)
- Polarization mismatch and antenna misalignment
- Cable, connector, and duplexer insertion losses
For real network design, FSPL should be only one part of a complete RF link budget and fade margin analysis.
Quick FAQ
Is lower FSPL better?
Yes. Lower path loss generally means stronger received signal and better link reliability.
Can FSPL be negative?
No, not for practical positive distance and frequency values. It represents attenuation.
Which unit choice is best?
Any unit is fine as long as conversions are handled correctly. This calculator performs those conversions automatically.