Free Space Path-loss & RSSI Calculator

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Link preset:

Presets are editable planning examples, not measured site profiles.

Carrier frequency:

Use the actual channel centre, such as 868 MHz, 915 MHz, 2.412 GHz, or 5.8 GHz.

Frequency unit

Path distance:

Enter a positive antenna-to-antenna distance.

Distance unit

TX power:

Use conducted radio power, not EIRP.

dBm

TX antenna gain:

dBi

RX antenna gain:

dBi

Additional path loss:

Enter zero for a clean free-space reference.

RX sensitivity:

dBm

Target RSSI:

dBm

Fade reserve:

Check link assumptions. {{ workflowFeedback }}

TX feed loss:

RX feed loss:

Polarization mismatch:

Noise bandwidth:

Bandwidth unit

Receiver noise figure:

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Decision	Recommendation	Reason	Copy
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Range envelope

Where does predicted receiver power cross the current planning floor as path distance changes?

A radio link succeeds when enough signal reaches the receiver after every gain and loss in the path is counted. Transmit power and antenna gain add to the budget. Feed-line loss, free-space spreading, polarization mismatch, and other path losses subtract from it. The remaining receiver input is usually expressed as received signal strength indicator (RSSI) in dBm.

Free-space path loss describes ideal geometric spreading between unobstructed antennas. It rises with both frequency and distance: doubling either one adds about 6 dB of loss. That makes the equation a useful reference for line-of-sight planning, but buildings, terrain, foliage, rain, antenna pattern, reflections, cable condition, and interference can move a real measurement far away from the ideal result.

dBm: An absolute power level referenced to one milliwatt. More negative values represent weaker received power.
dB: A gain, loss, or difference between levels. It is added or subtracted in a link budget.
Receiver sensitivity: The weakest input at which the receiver can meet a stated performance condition.
Fade reserve: Extra signal above sensitivity kept for fading, obstructions, installation error, and changing conditions.

The planning floor should reflect the stricter of two requirements: the target RSSI, or receiver sensitivity plus the desired fade reserve. A link can sit above raw sensitivity and still miss the reserve needed for reliable service. Signal-to-noise ratio (SNR) adds another view by comparing predicted receiver power with thermal noise and receiver noise figure over the selected bandwidth.

Clear line of sight is not merely a thin ray between antennas. The first Fresnel zone forms a widening volume around that ray, and obstacles intruding into it can introduce diffraction loss even when the antenna endpoints remain visible. A common planning check is 60% clearance at the midpoint, where the first zone is widest for equal path halves.

How to Use This Tool:

Start from measured radio and antenna values whenever possible. Presets are editable examples, not site profiles.

Choose a Link preset or enter the carrier frequency and antenna-to-antenna distance. Use the actual channel centre and confirm MHz versus GHz and metres versus kilometres.
Enter conducted TX power, both antenna gains, feed losses, additional path loss, and polarization mismatch. Do not enter EIRP as TX power because antenna gain would then be counted twice.
Set receiver sensitivity, target RSSI, and fade reserve. The stricter value becomes the Planning floor.
Enter the receiver noise figure and noise bandwidth, then compare predicted RSSI, reserve, SNR, 60% Fresnel clearance, and the range crossing. Recheck any assumption before selecting antennas or declaring a link viable.

Interpreting Results:

Reserve is predicted RSSI minus the planning floor. Positive reserve means the idealized budget clears the chosen target; negative reserve is the extra gain or loss reduction needed to reach it. Link margin compares RSSI only with raw receiver sensitivity, so it can remain positive when the fade-reserve target has already been missed.

RF reserve interpretation bands
Result	Boundary	Meaning
Robust RF reserve	Reserve ≥ 20 dB	Strong modeled headroom above the planning floor
Healthy planning reserve	10 dB ≤ reserve < 20 dB	Useful modeled headroom, still subject to site conditions
Meets the planning floor	0 dB ≤ reserve < 10 dB	Passes the selected floor with limited headroom
Decodes but misses reserve	Reserve < 0 dB and link margin ≥ 0 dB	Above sensitivity but below the selected planning target
Below the receive floor	Link margin < 0 dB	Predicted receiver power is below sensitivity

Maximum range is the distance where the same idealized budget reaches the current planning floor. It is not a coverage promise. Before deployment, verify antenna heights, regulatory power limits, cable losses, Fresnel clearance, interference, and RSSI or packet performance from a site survey or field test.

Technical Details:

The calculation normalizes frequency to megahertz, distance to kilometres, and bandwidth to hertz. Gains and losses stay in decibels, so the link budget can be assembled by addition and subtraction. No terrain, obstruction, multipath, atmospheric, or regulatory propagation model is added beyond the user-entered additional loss.

Formula Core

For distance d in kilometres and frequency f in megahertz, ITU-R free-space attenuation is evaluated with the conventional 32.44 dB constant.

L_{FS} = 32.44 + 20 \log_{10} (d_{km}) + 20 \log_{10} (f_{MHz})

Receiver power adds conducted transmitter power and both antenna gains, then subtracts feed losses, additional path loss, polarization mismatch, and free-space loss.

P_{RX} = P_{TX} + G_{TX} + G_{RX} - L_{system} - L_{FS}

Total system loss includes both feed losses, additional path loss, and polarization mismatch. Transmit EIRP is also reported separately as conducted TX power plus TX antenna gain minus TX feed loss.

The planning floor is the larger of target RSSI and sensitivity plus fade reserve. Reserve and required gain follow directly from that floor.

\begin{matrix} P_{floor} & = & \max (P_{target}, P_{sensitivity} + F_{reserve}) \\ Reserve & = & P_{RX} - P_{floor} \end{matrix}

The maximum modeled range rearranges the free-space equation so receiver power equals the planning floor. Range therefore changes by a factor of ten for every 20 dB of budget change, with all other inputs fixed.

Fresnel and Noise Mechanisms

Wavelength is the exact speed of light divided by frequency. For equal path halves, the first Fresnel-zone radius at the midpoint is the square root of wavelength times total path length divided by four; the displayed clearance is 60% of that radius.

r_{1} = \sqrt{\frac{λ \times D}{4}}

Thermal noise uses the exact Boltzmann constant, a reference temperature of 290 K, and the selected noise bandwidth. Receiver noise figure is then added in decibels, and SNR is predicted RSSI minus that noise floor.

N_{dBm} = 10 \log_{10} (\frac{k \times 290 \times B}{0.001}) + NF

B is bandwidth in hertz and NF is receiver noise figure in dB. Narrower bandwidth lowers thermal noise, but it does not model external interference or guarantee that the selected radio mode can operate in that bandwidth.

Accuracy Notes:

Free-space attenuation assumes ideal unobstructed propagation and matched inputs. The largest practical errors usually come from the environment or an incorrect budget entry, not from numeric precision.

Antenna gain must be stated for the intended direction and polarization; feed loss should include connectors and the actual cable run.
Additional path loss is a manual allowance, not an obstacle, terrain, rain, foliage, or multipath prediction.
Receiver sensitivity depends on modulation, data rate, coding, bandwidth, and the vendor's performance criterion.
Predicted SNR includes thermal noise and noise figure but excludes site interference.
Regulatory EIRP and band-use limits must be checked separately for the country and service.

Worked Examples:

A short 2.4 GHz planning path

At 2.412 GHz over 50 m, with 18 dBm conducted TX power, 3 dBi antennas at both ends, 1 dB feed loss per end, and 8 dB additional loss, free-space loss is about 74.1 dB and predicted receiver power is about −60.1 dBm. A −72 dBm sensitivity plus 6 dB fade reserve creates a −66 dBm planning floor, leaving about 5.9 dB reserve. The link meets the modeled floor but remains in the limited-headroom band, so the next check is the actual site path and measured signal, not the idealized 99 m range crossing alone.

References:

Recommendation ITU-R P.525-5: Calculation of free-space attenuation, International Telecommunication Union, November 2024.
Recommendation ITU-R P.526-16: Propagation by diffraction, International Telecommunication Union, November 2025.
Boltzmann constant, National Institute of Standards and Technology.