Free Space Path Loss Calculator (FSPL in dB) & Link Budget
By Santaji GadepatilinUpdated October 30, 2026
Free-space path loss is how much a radio signal weakens as it spreads out over distance in a clear line of sight. This free space path loss calculator gives the FSPL in dB and a simple link budget — received power from transmit power and antenna gains — for LoRaWAN sensor networks, wireless transmitters, Wi-Fi and point-to-point links.
How to use the free space path loss calculator
Enter the distance and frequency of the link. The calculator returns the free-space path loss in dB. Add the transmit power and both antenna gains to get the received power, and compare it with the receiver sensitivity. To find the maximum free-space range, clear the distance and enter the minimum received power.
Free-space path loss formula
FSPL = 20 log₁₀(4π d f ÷ c)
FSPL (dB) = 20 log₁₀ dkm + 20 log₁₀ fMHz + 32.44
Pr = Pt + Gt + Gr − FSPL
Example
A LoRa sensor at 868 MHz, 2 km from the gateway, 14 dBm transmit power and 2 dBi antennas at both ends:
- FSPL = 20 log₁₀(4π × 2000 × 868 × 10⁶ ÷ 299,792,458) = 97.24 dB
- Pr = 14 + 2 + 2 − 97.24 = -79.24 dBm — far above LoRa sensitivity, leaving a large margin for obstacles
Free-space path loss by distance and frequency
| Distance | 433.92 MHz | 868 MHz | 2.44 GHz |
|---|---|---|---|
| 10 m | 45.2 dB | 51.2 dB | 60.2 dB |
| 100 m | 65.2 dB | 71.2 dB | 80.2 dB |
| 500 m | 79.2 dB | 85.2 dB | 94.2 dB |
| 1 km | 85.2 dB | 91.2 dB | 100.2 dB |
| 2 km | 91.2 dB | 97.2 dB | 106.2 dB |
| 5 km | 99.2 dB | 105.2 dB | 114.2 dB |
| 10 km | 105.2 dB | 111.2 dB | 120.2 dB |
| 20 km | 111.2 dB | 117.2 dB | 126.2 dB |
Notes
- FSPL uses isotropic antennas; real antenna gains are added separately in the link budget.
- Keep the first Fresnel zone clear — at 868 MHz over 2 km its radius at mid-path is about 13 m, so mount antennas high.
- Convert dBm to milliwatts with the dBm to watts calculator.
Frequently asked questions
What is the free-space path loss formula?
FSPL = 20 log₁₀(4π d f ÷ c). In practical units, FSPL (dB) = 20 log₁₀ d (km) + 20 log₁₀ f (MHz) + 32.44.
How much does path loss increase with distance?
6 dB every time the distance doubles, and 20 dB for every tenfold increase. Doubling the frequency also adds 6 dB.
Why is real-world loss higher than FSPL?
FSPL assumes clear line of sight with an unobstructed Fresnel zone. Walls, terrain, foliage, rain and multipath add tens of dB, so leave a fade margin of 10–20 dB or more.
How do I check whether a link will work?
Compare the received power with the receiver's sensitivity. For example, LoRa at SF12 can decode about −137 dBm, while Wi-Fi at high data rates needs about −65 to −70 dBm.