Use this tool to calculate free space path loss (FSPL) in dB. Optional link budget fields estimate received power.
Optional Link Budget Inputs
What is free space path loss?
Free space path loss (FSPL) is the signal attenuation that occurs as a radio wave spreads through empty space. Even with no walls, rain, foliage, or interference, the signal weakens over distance because its energy is distributed over a larger and larger area.
Engineers use FSPL as a baseline for wireless design: Wi-Fi links, microwave backhaul, satellite communication, telemetry systems, drone control links, and more. It is often one of the first calculations in a link budget.
FSPL formula used in this calculator
This calculator uses the standard equation:
FSPL (dB) = 20·log10(d) + 20·log10(f) + 32.44
where d is distance in kilometers and f is frequency in MHz.
The constant 32.44 handles unit scaling and the speed of light conversion.
Why dB?
Decibels simplify very large and very small power ratios into manageable values and allow additive math in link budgets: gains add, losses subtract.
How to use the calculator
- Enter the link distance and choose the unit.
- Enter frequency and choose the unit.
- Click Calculate FSPL.
- Optional: enter transmitter power and antenna gains to estimate received signal level.
Interpreting results
FSPL value
Higher FSPL means more attenuation. FSPL rises with both distance and frequency. Doubling distance adds roughly 6 dB loss. Doubling frequency also adds roughly 6 dB.
Wavelength
The calculator also reports wavelength (λ = c / f). Wavelength helps with antenna sizing,
propagation intuition, and line-of-sight considerations.
Estimated received power (optional)
If you provide transmitter power and antenna gains, the tool computes:
Pr (dBm) = Pt + Gt + Gr - FSPL - Lmisc
This is a simplified free-space estimate and does not include fading, polarization mismatch, terrain diffraction, multipath, or atmospheric absorption.
Worked example
Suppose a 5 GHz point-to-point link spans 2 km:
- Distance: 2 km
- Frequency: 5 GHz (5000 MHz)
- Transmit power: 23 dBm
- TX gain: 16 dBi
- RX gain: 16 dBi
- Extra losses: 2 dB
FSPL is about 112.44 dB, and estimated received power is around -59.44 dBm. If your receiver sensitivity at target data rate is below this level (for example -70 dBm), the link may be viable with margin.
Limitations of free space models
FSPL assumes ideal unobstructed propagation and isotropic spreading. Real deployments often deviate from this model.
- Obstacles and partial Fresnel zone blockage increase losses.
- Multipath fading can create deep short-term signal dips.
- Rain and atmospheric gases can matter at higher microwave/mmWave bands.
- Cable loss, connector loss, and implementation loss reduce delivered power.
- Interference and noise floor determine usable throughput, not just signal strength.
Practical design tips
- Keep clear line of sight and protect Fresnel zone clearance.
- Use directional antennas for long links to improve link budget.
- Prefer lower frequencies when penetration or diffraction is important.
- Build fade margin into designs rather than targeting minimum sensitivity.
- Validate with field measurements after planning.