The Formula

FSPL(dB) = 20 log10(d) + 20 log10(f) + 32.4478

d in km, f in MHz; use positive distance and frequency.

This is the kilometre/MHz form of the calculator's existing metre/Hz equation. The commonly rounded 32.44 constant gives slightly different final decimals. Free-space basic transmission loss is described in ITU-R P.525.

Worked examples: a 1 km free-space path

Isotropic free-space path loss, without antenna gains or feeder losses
Frequency	Distance	FSPL
900 MHz	1 km	91.533 dB
1000 MHz	1 km	92.448 dB
2400 MHz	1 km	100.052 dB
3500 MHz	1 km	103.329 dB

At 1 GHz over 1 km, enter 1000 MHz and 1 km: the result is 92.448 dB. Doubling either distance or frequency adds about 6.02 dB when the other input is fixed. Changing a unit selector changes how the numeric input is interpreted; check both the number and unit.

What this RF path loss estimate leaves out

FSPL describes geometric spreading in the far field with an unobstructed line of sight. It does not model terrain, buildings, foliage, multipath, rain or atmospheric absorption. It is not a prediction of indoor coverage or a guarantee that a radio link will work. Near-field conditions and insufficient Fresnel-zone clearance need a different analysis.

For received power, start with transmitter power, add antenna gains, and subtract FSPL, feeder loss and other losses. The link budget calculator also compares the result with receiver sensitivity and your chosen fade margin.

Calculation reference

FSPL = 20 log₁₀(4πdf/c), approximately 32.448 + 20 log₁₀(d_km) + 20 log₁₀(f_MHz).

Variables and units

Loss is dB; d is distance in metres, f is frequency in Hz and c is speed of light in m/s in the first form.

Worked example

At 900 MHz over 1 km, free-space path loss is approximately 91.533 dB.

Assumptions and limitations

Assumes far-field unobstructed propagation between isotropic references. Terrain, clutter, Fresnel obstruction, atmospheric loss, polarisation and fading need separate treatment.

Reference: ITU-R P.525 free-space attenuation. Cite this calculator using its permanent page URL and your access date.