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Free Space Path Loss Calculator

Compute FSPL using the Friis transmission equation for any frequency and distance. Get received power, dB path loss, and maximum range for your RF link.

// Signal Parameters
dBm
dBi
dBi
dB
// RX Sensitivity (optional)
dBm
⚠ Check inputs.
// Path Loss Results
Path Loss
Wavelength λ —mm
Free-Space Path Loss (FSPL) —dB
Total Path Loss —dB
Received Signal
EIRP —dBm
Received Power (PRX) —dBm
Received Power (linear) —nW
Link Margin vs Smin —dB
Range Calculations
Max range at Smin —m
FSPL at 1 km —dB
FSPL at 10 km —dB
FSPL doubles every 6dB per 2× distance
FSPL (dB) = 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c)
            = 20·log₁₀(dkm) + 20·log₁₀(fMHz) + 32.44 dB
            = 20·log₁₀(dm) + 20·log₁₀(fGHz) + 92.44 dB

Friis: PRX (dBm) = PTX + GTX + GRX − FSPL − Losses (all in dB/dBm)
// FSPL vs Distance — Multiple Frequencies
Current freq 900 MHz 2.4 GHz 5 GHz 28 GHz Your distance
// Quick Reference — FSPL at Common Distances
Distance FSPL (current f) PRX (dBm) vs Smin

Free Space Path Loss — The Friis Transmission Equation

Free-space path loss (FSPL) describes the attenuation of an electromagnetic wave as it propagates through free space in the absence of obstacles, atmospheric effects, or multipath. It is the starting point for every RF link budget calculation.

The FSPL Formula

FSPL (dB) = 20·log₁₀(4πd/λ) = 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c)

In practical units: FSPL (dB) = 20·log₁₀(d_km) + 20·log₁₀(f_MHz) + 32.44

Key Properties of FSPL

Frequency scaling: FSPL increases by 20 dB per decade of frequency — doubling the frequency adds 6 dB. This is why 28 GHz 5G mmWave links have ~20 dB more path loss than 900 MHz LTE at the same distance.

Distance scaling: FSPL increases by 20 dB per decade of distance — doubling the distance adds 6 dB (inverse square law).

Not absorption: FSPL is geometric spreading of the wavefront, not energy absorption. The receiving antenna captures only a small fraction of the expanding wavefront sphere.

The Friis Transmission Equation

P_RX (dBm) = P_TX (dBm) + G_TX (dBi) + G_RX (dBi) − FSPL (dB) − Losses (dB)

Antenna gains directly reduce the effective path loss — 10 dBi at each end reduces the path loss budget by 20 dB.

Common RF Link Examples

WiFi 2.4 GHz at 100 m: FSPL ≈ 80 dB. With 0 dBi antennas and 20 dBm TX: P_RX = −60 dBm. WiFi sensitivity is −90 dBm → 30 dB margin.

5G NR 28 GHz at 200 m: FSPL ≈ 121 dB. With 24 dBi phased arrays each side and 23 dBm TX: P_RX = −50 dBm — beamforming gain compensates for high mmWave path loss.

GPS L1 at 20,200 km: FSPL ≈ 182 dB. GPS satellites transmit at +47 dBm with 13 dBi antenna. P_RX ≈ −122 dBm — GPS receivers detect signals at −130 dBm.