How to use this calculator
- Enter or choose distance.
- Enter or choose distance unit.
- Enter or choose frequency.
- Enter or choose frequency unit.
- Enter or choose transmit power.
- The free-space path loss updates as you type. Use Copy result or Share to keep it.
Method
FSPL (dB) = 20 log₁₀(d) + 20 log₁₀(f) + 20 log₁₀(4π ÷ c) = 20 log₁₀(d km) + 20 log₁₀(f MHz) + 32.45; Received = Tx power + Tx gain + Rx gain − FSPL
- d
- distance in metres
- f
- frequency in hertz
- c
- 299,792,458 m/s
Worked example
- 20 log₁₀(0.1 km) = −20; 20 log₁₀(2,400 MHz) = 67.60.
- FSPL = −20 + 67.60 + 32.45 = 80.05 dB.
- Received = 20 + 2 + 2 − 80.05 = −56.05 dBm.
Over 100 m at 2.4 GHz, free-space loss is 80.05 dB, so a 20 dBm transmitter with 2 dBi antennas delivers about −56 dBm.
What the result means
Compare the received level with the receiver’s sensitivity for the data rate you want, and keep a fade margin of 10–20 dB for walls, bodies and multipath, which free space ignores.
Assumptions and limits
- Clear line of sight with the first Fresnel zone unobstructed, in the far field of both antennas.
- Walls, floors, foliage, rain and reflections are not included; indoor losses are usually much higher than free space.
Questions
Why does 5 GHz reach less far than 2.4 GHz?
Path loss rises 20 log₁₀(5 ÷ 2.4) ≈ 6.4 dB at the same distance with similar antennas, and walls absorb more at higher frequencies.
What is dBm?
Power relative to 1 milliwatt on a log scale: 0 dBm = 1 mW, 20 dBm = 100 mW, −60 dBm = one nanowatt (10⁻⁶ mW).