Calculate free space path loss, received signal power, link margin, and maximum range for RF links using the Friis transmission equation.
Free Space Path Loss (FSPL) describes the attenuation of a radio signal as it propagates through unobstructed space between a transmitter and receiver. It is the foundational building block of any RF link budget calculation, used in WiFi planning, cellular network design, satellite communications, and point-to-point microwave links.
FSPL increases with both distance and frequency — doubling the distance adds 6 dB of loss, and doubling the frequency also adds 6 dB. This is why higher-frequency bands like 5 GHz WiFi or millimeter-wave 5G have shorter range than lower-frequency bands at the same power level.
This FSPL Calculator computes the path loss in decibels, the received signal power using the Friis transmission equation, the first Fresnel zone radius for line-of-sight clearance, and the maximum range for a given receiver sensitivity. Enter your frequency, distance, transmit power, and antenna gains to get a complete link budget summary. The reference table shows FSPL at 1 km for common frequency bands from AM radio to 60 GHz.
Use this calculator to estimate theoretical path loss, received power, and link margin before you add real-world losses such as foliage, diffraction, or building penetration. It is a fast first-pass check for whether a link budget is even plausible in ideal conditions. That makes it useful for screening candidate frequencies or distances early in the design process.
FSPL (dB) = 20 log₁₀(d) + 20 log₁₀(f) + 32.44 where d = distance in km, f = frequency in MHz Received Power: Pr = Pt + Gt + Gr − FSPL (all in dB/dBm/dBi) Fresnel Radius: r = 17.32 × √(d / (4f)) at midpoint
Result: FSPL = 100.0 dB, Received Power = −80.0 dBm
At 2.4 GHz over 1 km with 20 dBm transmit power and unity-gain antennas, the signal arrives at −80 dBm — above most WiFi receiver sensitivities.
Free-space path loss is the geometric spreading loss for a radio signal in an ideal unobstructed environment. It gives you the theoretical floor for path loss before terrain, clutter, weather, polarization mismatch, and hardware losses push the real number higher.
At the same distance, higher-frequency links suffer higher free-space loss. That does not automatically make them unusable, but it means they often need more antenna gain, shorter paths, or tighter alignment than lower-frequency systems.
FSPL is useful because it turns a qualitative radio problem into a first-pass numbers check. If the link already fails in free space, no installation trick will save it. If it passes comfortably, the next step is adding realistic fade and obstruction margins.
FSPL assumes a perfect unobstructed path with no extra environmental losses. Real-world losses from buildings, vegetation, rain, polarization mismatch, and multipath are additional and often dominate the real link budget.
Because FSPL scales with the square of frequency, so doubling frequency adds about 6 dB in the logarithmic form. Higher frequencies also correspond to smaller effective apertures for the same physical antenna assumptions.
The difference between received power and the minimum receiver sensitivity. A margin of 10–20 dB is typical for reliable links. That extra headroom absorbs fading, weather loss, and other non-ideal effects that the free-space model does not include.
An ellipsoidal region around the line of sight. At least 60% of the first Fresnel zone should be clear of obstructions for a reliable link. A path can look visually clear and still fail if Fresnel clearance is poor.
Higher antenna gain increases effective radiated power on the transmit side and concentrates received energy on the receive side, which improves the link budget without changing transmitter output power. That is why antenna choice can rescue a marginal link even when the transmitter itself does not change.
FSPL is the theoretical minimum path loss in free space. Actual path loss includes additional factors like diffraction, reflection, and absorption. In practice, FSPL is the clean baseline and real path loss is usually worse.