Free Space Path-loss & RSSI Calculator

{{ formatDbm(rxPower, 1) }} dBm

{{ verdict.badgeLabel }} {{ formatFrequencyLabel(freqMHz) }} {{ formatDistanceFromKm(distanceKm, distUnit) }} FSPL {{ formatDb(fspl, 1) }} dB EIRP {{ formatDbm(eirp_dBm, 1) }} dBm {{ reserveBadge }}

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.

Section	Signal	Value	Detail
Planning snapshot	{{ tile.label }}	{{ tile.value }}	{{ tile.detail }}
Verdict	{{ verdict.title }}	Link status	{{ verdict.detail }}
What to do next	{{ item.title }}	{{ item.badge }}	{{ item.detail }}
Model assumption	Assumption {{ idx + 1 }}	Input model	{{ note }}

Stage	Delta	Running	Why it matters	Copy
{{ row.label }}	{{ row.deltaDisplay }}	{{ row.runningDisplay }}	{{ row.note }}

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.

Embed:

Customize

Include current inputs

Size

Advanced

Width

Height

Aspect ratio

Max height

Collapsible embed

Allow fullscreen

Referrer policy

Sandbox tokens

A radio path that works on a bench can fail in the field because the received signal is only one part of the link. Distance spreads the wave, frequency changes how quickly the signal weakens, antennas push energy in useful directions, cables and connectors take some power back, and the receiver still needs enough signal above its own decode floor. A path-loss and received signal strength indicator estimate puts those pieces into one dB budget before hardware is mounted.

Free-space path loss, often shortened to FSPL, is the clean reference case. It assumes an unobstructed path between ideal antennas and calculates how much signal is lost as the wavefront expands. Real installations rarely stay that clean. Walls, foliage, rain fade, polarization mismatch, body blocking, terrain, reflections, and interference can all consume the reserve that looked comfortable on paper.

RSSI is normally shown in dBm, so values are often negative. A value near -55 dBm is much stronger than -85 dBm, and a few dB can matter when the receiver is close to its threshold. Because dB units are logarithmic, the link budget can add gains and subtract losses directly. That makes it easier to see whether a change in antenna gain, cable loss, distance, or frequency is large enough to matter.

Distance and frequency set the baseline spreading loss. Doubling either one adds about 6 dB of free-space loss.
Antenna gain and feed loss decide how much useful power leaves the transmit side and reaches the receiver input.
Receiver sensitivity and target RSSI define the receive floor. A planning target can be stricter than the bare decode threshold.
Fade reserve is extra margin for changing weather, obstruction, alignment, and site variation.
Fresnel clearance and noise bandwidth explain why a path can have enough RSSI but still behave poorly.

Transmit budget, path loss, Fresnel space, and receive floor in a radio link budget

A common mistake is treating a positive reserve as a guarantee. It only means the entered assumptions close under the model. A survey can still reveal an obstruction in the Fresnel zone, a cable loss that was not counted, an antenna pattern pointed the wrong way, or a noise floor that leaves less signal-to-noise ratio than expected. The best use of an early link budget is to find weak assumptions before buying or climbing hardware.

How to Use This Tool:

Start with the closest preset, then replace the assumptions that differ from the actual radios, antennas, path, and required receive floor.

Choose Preset first. Use Custom / manual when none of the LoRa, telemetry, Wi-Fi, bridge, or backhaul presets matches the path.
Set Frequency and Distance with the right units. Results appear only when both values are positive, because FSPL cannot be computed from a zero or blank path.
Enter TX power, TX antenna gain, RX antenna gain, and Additional path loss. Put wall loss, foliage, rain fade, body blocking, or clutter into additional loss rather than hiding it in the antenna fields.
Open Advanced for TX cable loss, RX cable loss, Polarization mismatch, Noise figure, and Noise bandwidth. Noise and SNR stay unmodeled until bandwidth is above 0.
Set RX sensitivity, Target RSSI, and Fade reserve. If both a target and a sensitivity-plus-reserve floor are present, the calculator uses the stricter dBm value as the Planning floor.
Read the summary and Link Verdict first. If the verdict says Free-space receive estimate, add a receive floor before using reserve, planned range, or the gain-gap view for deployment decisions.
Use Budget Ladder to find which gain or loss changed the RSSI, then compare Range Envelope, Band Comparison, and Gain Gap Map when the question is distance, RF band choice, or extra system gain.

For a first pass, make the numbers slightly pessimistic: include known feed losses, add realistic site loss, and use a receive floor that reflects the service quality you need rather than only the lowest decode sensitivity.

Interpreting Results:

Predicted receiver input is the modeled RSSI after transmitter power, antenna gains, feed losses, free-space path loss, additional path loss, polarization mismatch, and receive-side losses. Planning floor is the stricter of Target RSSI and RX sensitivity plus Fade reserve when those values are available.

Available reserve is the main planning number. At 20 dB or more, the verdict becomes Robust RF reserve. From 10 dB to less than 20 dB, it becomes Healthy planning reserve. From 0 dB to less than 10 dB, it only Meets the planning floor, so a small amount of unmodeled loss can erase the margin.

A below-floor result does not always mean the receiver cannot decode. If the modeled RSSI still clears bare sensitivity but misses the reserve target, the verdict changes to Decodes but misses reserve. Treat that as a warning for outdoor, mobile, obstructed, or high-availability links, not as a comfortable pass.

Check Fresnel midpoint and Noise / SNR before trusting a strong-looking RSSI. The midpoint Fresnel value is only a clearance cue, and SNR depends on the bandwidth and noise figure assumptions. Field measurements and path-profile checks remain necessary for important links.

Technical Details:

Free-space attenuation is the reference loss between isotropic antennas in an ideal unobstructed path. The practical point-to-point form uses distance in kilometers and frequency in megahertz. Higher frequency and longer distance both enter through base-10 logarithms, so each doubling adds roughly 6 dB.

The link budget is an accounting problem in dB units. Transmit power starts in dBm, antenna gains add dB, cable and site losses subtract dB, and the final received power is compared with a required receive floor. The receive floor is expressed in dBm, so a less negative value such as -65 dBm is stricter than -85 dBm.

Formula Core:

The core calculation combines free-space loss, system gains and losses, the stricter receive floor, Fresnel midpoint radius, and optional thermal-noise math.

\begin{array}{lcl} L_{FSPL} & = & 32.44 + 20 \log_{10} (d_{km}) + 20 \log_{10} (f_{MHz}) \\ EIRP & = & P_{TX} + G_{TX} - L_{TX cable} \\ P_{RX} & = & EIRP + G_{RX} - L_{RX cable} - L_{additional} - L_{polarization} - 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}

Variable meanings for the RF path-loss calculation
Symbol	Meaning	Unit or source
`d`	Path distance	Entered as meters or kilometers, converted as needed
`f`	Carrier frequency	Entered as MHz or GHz, converted to MHz for FSPL
`EIRP`	Transmit-side radiated budget before free-space loss	dBm
`Prx`	Predicted receiver input power, displayed as RSSI	dBm
`Pfloor`	Required receive floor used 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 about 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 the predicted receiver input is about -60.1 dBm. A sensitivity of -72 dBm with 6 dB fade reserve gives a -66 dBm floor, which is stricter than the -67 dBm target RSSI, leaving about 5.9 dB of reserve.

Link verdict thresholds used by the calculator
Verdict	Condition	Planning meaning
`Free-space receive estimate`	No receive floor is set	RSSI is modeled, but reserve and deployment judgment are incomplete
`Robust RF reserve`	Reserve is at least 20 dB	Large modeled headroom remains after the chosen floor
`Healthy planning reserve`	Reserve is at least 10 dB and less than 20 dB	Workable headroom, but site losses still deserve attention
`Meets the planning floor`	Reserve is at least 0 dB and less than 10 dB	The budget closes, but small unmodeled losses can break the plan
`Decodes but misses reserve`	Reserve is negative, but RSSI is still above bare sensitivity	The receiver may demodulate in clean conditions without the requested margin
`Below the receive floor`	RSSI is below the effective floor and does not clear bare sensitivity	The entered path needs more gain, less loss, shorter distance, or a different band

The midpoint Fresnel radius uses the wavelength and total path length, then reports 60% of that value as a practical clearance target. The range and gain-gap charts reuse the same budget while sweeping distance, and the band comparison keeps every gain and loss fixed so frequency is the only changed variable.

Accuracy Notes:

Free-space RSSI is an estimate, not a site survey. Treat these limits as part of the result:

Terrain, diffraction, reflections, moving obstructions, antenna pattern nulls, 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.
Radio-reported RSSI can differ by chipset, calibration method, channel width, automatic gain control, and measurement timing.
Noise floor and SNR depend on the noise bandwidth and noise figure you enter; bandwidth left at 0 disables that part of the model.

Worked Examples:

A short 2.4 GHz Wi-Fi client path using the default values gives FSPL near 74.1 dB and Predicted receiver input near -60.1 dBm. The Planning floor becomes -66.0 dBm from sensitivity plus fade reserve, so Available reserve is about 5.9 dB. The Link Verdict is Meets the planning floor, which is useful for a first check but not a generous outdoor margin.

A 868 MHz LoRa field-node preset over 2 km is a different kind of path. With low antenna gain, 2 dB of additional path loss, -123 dBm sensitivity, -120 dBm target RSSI, and 10 dB fade reserve, predicted RSSI is about -87.2 dBm. The stricter floor is -113.0 dBm from sensitivity plus reserve, leaving roughly 25.8 dB of Available reserve and a Robust RF reserve verdict under the model.

A 5.8 GHz bridge can flip from workable to failing when hidden losses are added. At 3 km with 18 dBi antennas on both ends, the clean preset lands around -60.3 dBm with about 4.7 dB of reserve against a -65 dBm floor. Add 18 dB of obstruction loss and 3 dB of polarization mismatch, and the modeled RSSI drops near -81.3 dBm. The Gain Gap Map then shows roughly 16.3 dB of extra system gain needed, and the verdict falls to Below the receive floor.

FAQ:

Why does higher frequency reduce the predicted RSSI?

The FSPL equation includes 20 log10(frequency), so a higher band has more free-space loss at the same distance. The Band Comparison chart keeps the rest of the budget fixed to show that frequency effect.

Should I use receiver sensitivity or target RSSI?

Use RX sensitivity for the bare decode threshold and Target RSSI for the service level you want to plan around. When both are entered, the calculator uses the stricter receive floor.

Why do noise floor and SNR sometimes show as not modeled?

Noise / SNR requires Noise bandwidth above 0 and a Noise figure. If bandwidth is 0, the RSSI budget still works but thermal-noise math is skipped.

What should I enter as additional path loss?

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

Can a positive reserve guarantee the 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.
Noise Figure: Noise Concepts, Keysight Technologies.