Free Space Path-loss & RSSI Calculator

Preset:

Choose Custom to keep manual values, or pick a LoRa, telemetry, Wi-Fi, or bridge baseline.

Frequency:

Use the actual channel center, for example 868 MHz, 915 MHz, 2.412 GHz, or 5.8 GHz.

Distance:

Enter the antenna-to-antenna path in meters or kilometers; choose m for short links such as 50 m.

TX power:

Enter transmitter power in dBm, for example 14 for LoRa or 20 for Wi-Fi.

dBm

TX antenna gain:

Use antenna gain in dBi from the data sheet; negative values can model inefficient antennas.

dBi

RX antenna gain:

Enter receive antenna gain in dBi; use 0 for a small omnidirectional whip.

dBi

Additional path loss:

Use dB penalties from surveys or estimates, for example 8 for indoor wall loss.

RX sensitivity:

Enter receiver sensitivity in dBm, usually negative, such as -123 or -72.

dBm

Target RSSI:

Optional dBm floor for deployment quality, for example -120 for LoRa or -67 for Wi-Fi.

dBm

Fade reserve:

Enter 0-60 dB of cushion; 10 dB is the default planning reserve.

Uses free-space attenuation by default. Put wall loss, foliage, rain fade, body loss, or other non-line-of-sight penalties into Additional path loss.

TX cable loss:

Enter total transmit-side insertion loss in dB; 0 means no modeled feed loss.

RX cable loss:

Enter receive-side insertion loss in dB between antenna and radio; 0 disables it.

Polarization mismatch:

Enter mismatch loss in dB; keep 0 for aligned antennas.

Noise figure:

Enter receiver NF in dB; 5-7 dB is typical for many radio front ends.

Noise bandwidth:

Enter channel bandwidth and unit, for example 125 kHz for LoRa or 20 MHz for Wi-Fi.

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Predicted RSSI is swept over distance. If you set both sensitivity and target RSSI, the stricter floor is used for planning reserve.

Comparison keeps distance, antenna gains, and all entered losses fixed so you can see what the band choice alone is doing to receive power.

Reference floor: {{ referenceFloorLabel }}. Values below 0 dB mean reserve is available; values above 0 dB show how much extra system gain is still needed.

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A radio link can look generous in a catalog and still arrive weak at the receiver. The signal leaves a transmitter with a known power, gains or loses strength through antennas and feed lines, spreads across distance, and then has to clear a receiver threshold while the environment keeps changing. Link-budget work gives those pieces one shared unit, decibels, so a 3 dB cable loss, a 6 dB antenna gain, or a 20 dB wall penalty can be compared directly.

Free-space path loss is the clean reference case for that budget. It models an unobstructed path between ideal antennas and asks how much power is lost as the wavefront expands. Real paths add clutter, terrain, moisture, reflections, polarization errors, foliage, body blocking, and interference. The reference value is still useful because it separates the unavoidable distance-and-frequency loss from site losses that need a survey, a drawing, or a pessimistic allowance.

RSSI: Received signal strength indicator, usually expressed in dBm. A less negative number, such as -55 dBm, is stronger than -85 dBm.
Receiver sensitivity: The bare decode threshold published for a radio mode, modulation, or data rate.
Target RSSI: A planning floor chosen for service quality, often stricter than the lowest decode threshold.
Fade reserve: Extra headroom above sensitivity to absorb fading, alignment error, weather, and other losses that are not stable.
Fresnel clearance: Open space around the visual line of sight. An obstruction can hurt a link even when the endpoints can see each other.

Transmit budget, path loss, Fresnel space, and receive floor in an RF link budget

The same distance can behave very differently at different frequencies. A short indoor Wi-Fi hop may lose enough power through walls to need a stricter target RSSI, while a long LoRa or telemetry path can close because narrow bandwidth and low data rate allow very sensitive receivers. Directional outdoor bridges add antenna gain, but they also depend on clean alignment and a Fresnel zone that is not clipped by rooftops, trees, or terrain.

A positive margin is not a guarantee. It means the entered assumptions close under the model. A practical plan still checks antenna patterns, legal transmit limits, feed-line losses, mounting height, channel noise, the full path profile, and the installed RSSI once the equipment is on site.

How to Use This Tool:

Start from a preset that resembles the link, then replace every assumption that differs from the radios, antennas, path, and required receive level.

Choose Preset. Use Custom / manual for paths that do not match the LoRa, telemetry, Wi-Fi, bridge, or backhaul baselines.
Enter Frequency and Distance with the correct units. A zero or blank value prevents FSPL, RSSI, Fresnel, and range results from being meaningful.
Set TX power, TX antenna gain, RX antenna gain, and Additional path loss. Put walls, foliage, rain fade, body blocking, clutter, and other site penalties into the loss field instead of hiding them in antenna gain.
Open Advanced when feed lines, polarization mismatch, receiver noise figure, or channel bandwidth matter. Noise / SNR stays unmodeled until Noise bandwidth is above 0.
Set RX sensitivity, Target RSSI, and Fade reserve. When both target RSSI and sensitivity plus reserve are present, the stricter dBm value becomes the Planning floor.
Read the summary and Link Verdict. If the verdict remains Free-space receive estimate, add a receive floor before using reserve, planned range, or gain-gap results for deployment decisions.
Use Budget Ladder to audit gains and losses, then check Range Envelope, Band Comparison, or Gain Gap Map when the next decision is distance, band choice, or added system gain.

Interpreting Results:

Predicted receiver input is the modeled RSSI after transmitter power, antenna gains, feed losses, site loss, polarization mismatch, and FSPL are combined. Planning floor is the receive level that the link must clear, using the stricter of Target RSSI and RX sensitivity plus Fade reserve when both are available.

RF reserve verdict boundaries
Verdict	Boundary	What to do next
Robust RF reserve	Reserve is at least 20 dB	Still confirm Fresnel clearance and real site loss.
Healthy planning reserve	Reserve is at least 10 dB and less than 20 dB	Workable for many paths, but survey assumptions matter.
Meets the planning floor	Reserve is at least 0 dB and less than 10 dB	Treat small unmodeled losses as deployment risks.
Decodes but misses reserve	RSSI clears bare sensitivity but not the planning floor	Do not treat a clean-condition decode as a reliable margin.
Below the receive floor	RSSI is below the effective floor	Shorten the path, lower loss, add gain, or reassess the band.

The strongest-looking RSSI is not always the best final answer. Fresnel midpoint can reveal a clearance problem, and Noise / SNR can show that the chosen bandwidth and noise figure leave less useful signal separation than the power budget suggests.

For important links, compare the modeled result with an actual site survey or installed RSSI reading. A field reading that is weaker than the model usually points to uncounted obstruction, feed loss, antenna alignment, polarization mismatch, or a noisier channel than assumed.

Technical Details:

Free-space attenuation is the point-to-point reference loss between ideal antennas in an unobstructed path. In practical radio-link units, distance is expressed in kilometers and frequency in megahertz. The logarithmic form means a doubling of distance adds about 6 dB of loss, and a doubling of frequency adds another 6 dB when every other factor is unchanged.

The receive budget stays in dB units so gains and losses can be added directly. Transmit power begins in dBm, antenna gains add dB, cable and site losses subtract dB, and the final receive power is compared with a required floor. Less negative dBm values are stricter receive requirements, so -65 dBm is harder to satisfy than -85 dBm.

Formula Core:

The main equations cover FSPL, effective radiated budget, predicted RSSI, the planning floor, reserve, midpoint Fresnel radius, thermal noise, and SNR.

\begin{array}{lcl} L_{FSPL} & = & 32.44 + 20 \log_{10} (d_{km}) + 20 \log_{10} (f_{MHz}) \\ EIRP & = & P_{TX} + G_{TX} - L_{TX feed} \\ P_{RX} & = & EIRP + G_{RX} - L_{RX feed} - L_{site} - L_{pol} - L_{FSPL} \\ P_{floor} & = & \max (P_{target}, P_{sensitivity} + M_{fade}) \\ Reserve & = & P_{RX} - P_{floor} \\ r_{1 mid} & = & \sqrt{\frac{λ d_{m}}{4}} \\ N & = & - 174 + 10 \log_{10} (B_{Hz}) + NF \\ SNR & = & P_{RX} - N \end{array}

RF calculation variable meanings
Symbol	Meaning	Unit or source
`d`	Radio path distance	Meters or kilometers, converted to km for FSPL
`f`	Carrier frequency	MHz or GHz, converted to MHz for FSPL
`EIRP`	Transmit-side radiated budget before path loss	dBm
`Prx`	Predicted receiver input power	dBm, displayed as RSSI
`Pfloor`	Required receive level for reserve	dBm, from target RSSI or sensitivity plus fade reserve
`N`	Thermal noise floor after receiver noise figure	dBm, only when bandwidth is above 0

With the default 2.412 GHz Wi-Fi client values, a 50 m path gives 74.1 dB of FSPL. Transmit-side EIRP is 20.0 dBm, the receive-side contribution after antenna gain, feed loss, and additional path loss is -6.0 dB, so predicted RSSI is about -60.1 dBm. Sensitivity plus 6 dB fade reserve gives a -66.0 dBm floor, which is stricter than the -67.0 dBm target, leaving about 5.9 dB of reserve.

Range and gain-gap views reuse the same budget while sweeping distance or added gain. Band comparison keeps every gain and loss fixed while changing only frequency, so its main purpose is to expose the frequency term in FSPL rather than to promise real-world range in a noisier or more obstructed band.

Accuracy Notes:

The calculation is a planning estimate, not a site survey. Treat the result as a disciplined first pass and keep these limits visible:

Terrain, diffraction, reflections, antenna pattern nulls, weather variation, regulatory power limits, and interference are not solved by the FSPL equation.
The Fresnel value is a midpoint clearance cue, not a full terrain profile along the path.
Chipset RSSI readings can differ by calibration, automatic gain control, channel width, and reporting interval.
Noise floor and SNR depend on the entered bandwidth and noise figure; leaving bandwidth at 0 disables that math.
The calculation runs from the entered values in the browser and does not require a remote path lookup.

Advanced Tips:

Use the actual channel center, such as 868 MHz, 915 MHz, 2.412 GHz, or 5.8 GHz, when comparing bands or hardware data sheets.
Put antenna coax and connector loss in the feed-loss fields, and reserve Additional path loss for site effects such as walls or foliage.
Set Target RSSI to the service-quality floor you want, not merely the lowest decode sensitivity.
Keep Fade reserve higher for outdoor, obstructed, mobile, or weather-exposed links than for short clean bench tests.
Compare Range Envelope runs only when frequency, gains, losses, sensitivity, target RSSI, and fade reserve use the same assumptions.
Use Band Comparison as a frequency effect check. It does not add band-specific noise, regulation, antenna pattern, or channel availability limits.

Worked Examples:

Short Wi-Fi client path

The default 2.4 GHz Wi-Fi client path over 50 m gives FSPL near 74.1 dB and Predicted receiver input near -60.1 dBm. The effective Planning floor is -66.0 dBm from sensitivity plus reserve, so Available reserve is about 5.9 dB. That clears the floor but leaves little room for extra wall loss or poor antenna placement.

LoRa field-node estimate

The LoRa 868 MHz field-node preset over 2 km, with 14 dBm transmit power, 1 dB feed loss on each side, 2 dB additional path loss, -123 dBm sensitivity, -120 dBm target RSSI, and 10 dB reserve, lands near -82.2 dBm RSSI. The stricter floor is -113.0 dBm from sensitivity plus reserve, leaving about 30.8 dB of reserve under the model.

Obstructed outdoor bridge

The 5.8 GHz outdoor bridge preset over 3 km starts near -60.3 dBm with about 4.7 dB of reserve against a -65.0 dBm target. Add 18 dB of obstruction loss and 3 dB of polarization mismatch, and predicted RSSI falls near -81.3 dBm. The path then needs about 16.3 dB of added system gain or loss reduction to meet the floor.

FAQ:

Why does a higher frequency lower the predicted RSSI?

FSPL includes a 20 log10(frequency) term. At the same distance and gain, a higher band has more free-space loss before site effects are added.

Should I enter receiver sensitivity or target RSSI?

Use RX sensitivity for the bare decode threshold and Target RSSI for the service level you want. When both are entered, the stricter receive floor controls reserve.

Why is noise floor or SNR shown as not modeled?

Noise / SNR needs Noise bandwidth above 0. With bandwidth left at 0, the RSSI budget still works but thermal-noise math is skipped.

What belongs in additional path loss?

Use Additional path loss for penalties outside clean free-space spreading, including walls, foliage, rain fade, body blocking, clutter, or other site-specific loss.

Can a positive reserve guarantee the installed link?

No. Positive Available reserve means the entered budget clears the selected floor. Fresnel obstruction, interference, wrong antenna alignment, or uncounted losses can still make the installed link worse.

Glossary:

FSPL: Free-space path loss, the baseline attenuation from distance and frequency in an ideal unobstructed path.
RSSI: Received signal strength indicator, represented here by predicted receiver input power in dBm.
EIRP: Effective isotropic radiated power, or transmit power after transmit antenna gain and transmit-side feed loss.
Fade reserve: Extra dB margin above sensitivity used to absorb fading, site variation, and unmodeled loss.
Fresnel zone: The ellipsoidal space around a radio path where obstructions can reduce signal quality even when visual line of sight exists.
Noise figure: The receiver noise penalty above an ideal thermal-noise reference, entered in dB for SNR estimation.

References:

Recommendation ITU-R P.525-5: Calculation of free-space attenuation, International Telecommunication Union, November 2024.
Wireless Mesh Access Points, Design and Deployment Guide: Site Preparation and Planning, Cisco, June 15, 2015.
Noise Figure Application, Keysight Technologies.