About the Free Space Path Loss Calculator
This free space path loss (FSPL) calculator works out how much a radio signal weakens as it spreads out between two antennas with a clear line of sight. Enter the distance and frequency to get the path loss in decibels, then add transmitter power, antenna gains, cable losses and the receiver sensitivity to turn it into a simple link budget: received signal level, link (fade) margin and the theoretical maximum range.
It is useful for planning point-to-point Wi-Fi bridges, LoRa and other IoT links, drone and FPV video links, amateur radio and microwave hops. The calculator also gives the radius of the first Fresnel zone at the midpoint of the path and the 60% clearance that is commonly kept free of trees, buildings and terrain.
Free space loss is a best case. It ignores ground reflections, obstructions, rain fade, atmospheric absorption and the curvature of the Earth, so real links usually need 10–20 dB of margin above the computed figure, and the maximum range shown is an upper bound, not a promise.
With the default inputs, the free space path loss is 120.05 dB. Change any value above to recalculate instantly.
How to use the free space path loss calculator
- 1Enter the path distance and choose km, metres or miles.
- 2Enter the operating frequency in MHz or GHz.
- 3Add transmitter power (dBm) and both antenna gains (dBi).
- 4Enter total cable and connector losses and the receiver sensitivity.
- 5Read the path loss, received level and link margin; aim for at least 10 dB of margin.
Formula and method
Free space path loss follows from the Friis transmission equation for isotropic antennas: the power density falls with the square of distance, and the effective capture area of an antenna shrinks with the square of wavelength. In decibels this becomes 20·log₁₀(4πdf/c), or the familiar shortcut 20·log₁₀(d in km) + 20·log₁₀(f in MHz) + 32.45.
The link budget adds the transmitter power and both antenna gains, subtracts cable/connector losses and the path loss, and compares the result with the receiver sensitivity to give the margin. The maximum free-space range is the distance at which the margin reaches zero: d_max = d × 10^(margin/20). The first Fresnel zone radius at the midpoint is ½·√(λd); keeping at least 60% of it clear avoids significant obstruction loss.
- d
- Distance between antennas (m)
- f
- Frequency (Hz)
- c
- Speed of light, 299,792,458 m/s
- λ
- Wavelength c/f (m)
- P_tx, P_rx
- Transmit power and received level (dBm)
- G_tx, G_rx
- Antenna gains (dBi)
- L
- Cable, connector and other fixed losses (dB)
Worked examples
2.4 GHz Wi-Fi bridge over 10 km
FSPL = 20·log₁₀(10) + 20·log₁₀(2400) + 32.45 ≈ 120.05 dB. With 20 dBm transmit power, two 15 dBi dishes and 2 dB of cable loss, the received level is 20 + 30 − 2 − 120.05 ≈ −72.05 dBm, about 12.9 dB above a −85 dBm receiver. The first Fresnel zone is 17.7 m in radius at mid-path.
915 MHz IoT link over 2 km
At 915 MHz the path loss over 2 km is about 97.7 dB. 20 dBm plus two 3 dBi antennas minus 2 dB of losses delivers −73.7 dBm, giving 26.3 dB of margin over a −100 dBm sensitivity — plenty of headroom for foliage and fading.
5.8 GHz link over 3 miles
Three miles is 4.828 km. At 5.8 GHz the loss is about 121.4 dB, but two 30 dBi dishes and 23 dBm transmit power leave a received level of −39.4 dBm — 40.6 dB above the −80 dBm sensitivity, enough for a high modulation rate.
Frequently asked questions
What is free space path loss?+
Free space path loss is the reduction in signal power between two isotropic antennas separated by distance d in a vacuum with no obstructions. It grows with the square of both distance and frequency, i.e. 20 dB per decade of either.
Why does path loss increase with frequency?+
The spreading of the wave does not depend on frequency, but the effective aperture of an antenna with fixed gain shrinks with the square of the wavelength. Higher frequencies therefore capture less power, which shows up as higher path loss.
How much link margin do I need?+
A common rule is at least 10 dB of fade margin for fixed line-of-sight links and 20 dB or more for links with foliage, moving terminals or long distances. Critical links are often designed for 25–30 dB.
What is the Fresnel zone and why does it matter?+
The first Fresnel zone is an ellipsoid around the direct path where reflected signals arrive nearly in phase. If obstacles block more than about 40% of it, extra loss appears even though there is still visual line of sight.
Is the maximum range figure realistic?+
It assumes perfect free space, so real-world range is usually much lower because of obstructions, ground reflections, interference and the curvature of the Earth (the radio horizon). Treat it as an upper bound.