Link parameters

Frequency f

Distance d

Frequency presets

Link budget (optional)

TX power P_TX

TX antenna gain G_TX

dBi

TX cable loss L_TX

dB

RX antenna gain G_RX

dBi

RX cable loss L_RX

dB

RX sensitivity S_min

dBm

Link diagram

Free-space path loss

Path Loss (FSPL)

—

Wavelength λ

—

Distance

—

Transmitter

TX Power

—

dBm

EIRP

—

dBm

TX Antenna Gain

—

dBi

Receiver

Received Power

—

dBm

Link Margin

—

RX Sensitivity

—

dBm

Key formulas

Free-space path loss

FSPL = 20·log 4π·d·f c

d in m · f in Hz · c = 3×10⁸ m/s · result in dB

Simplified (practical)

FSPL = 20·log(d_km)

+ 20·log(f_MHz) + 32.44

d in km · f in MHz

EIRP

EIRP = P_TX + G_TX − L_TX

all values in dBm / dBi / dB

Received power & margin

P_RX = EIRP − FSPL + G_RX − L_RX

M = P_RX − S_min

M = link margin above receiver sensitivity

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Free Space Path Loss — Complete Reference

Free-space path loss (FSPL) is the attenuation that a radio wave experiences as it propagates through a vacuum or clear atmosphere, with no obstructions, reflections, or absorption. It is the theoretical minimum loss any RF link will experience and serves as the baseline for all link budget calculations. Real-world links will always have additional losses from atmospheric absorption, multipath, vegetation, and building penetration on top of FSPL.

The FSPL Formula

Free-space path loss — exact form

FSPL (dB) = 20·log₁₀(4π·d·f / c)
= 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c)

d in metres · f in Hz · c = 299,792,458 m/s · result in dB

Simplified practical form (km and MHz)

FSPL (dB) = 20·log₁₀(d_km) + 20·log₁₀(f_MHz) + 32.44

Quick estimate: at 2.4 GHz over 100 m → 20·log(0.1) + 20·log(2400) + 32.44 = −20 + 67.6 + 32.44 = 80 dB

Why FSPL Increases with Both Distance and Frequency

The 20·log rule means FSPL doubles (increases by 6 dB) every time distance or frequency doubles. This has practical consequences:

Moving from 2.4 GHz to 5.8 GHz increases FSPL by ~7.7 dB for the same link — the 5 GHz Wi-Fi band always has shorter range than 2.4 GHz at the same power.
Doubling the link distance always costs 6 dB, regardless of frequency — this is why antenna height matters so much for long links.
Lower frequency bands (868 MHz, 433 MHz) are preferred for long-range IoT (LoRa) precisely because lower FSPL allows the same range with less TX power.

Link Budget

A link budget adds up all the gains and losses in a wireless link to determine whether the received signal will be above the receiver's sensitivity threshold:

Link budget equation

P_RX (dBm) = P_TX − L_TX + G_TX − FSPL + G_RX − L_RX

Link margin (dB) = P_RX − S_min

EIRP (dBm) = P_TX + G_TX − L_TX

P_TX = TX power · L_TX/RX = cable+connector losses · G_TX/RX = antenna gains · S_min = receiver sensitivity

Rule of thumb for link margin: A minimum margin of 10 dB is required for reliable operation. 20 dB provides good fade margin for links with environmental variability (rain, vegetation, multipath). Critical infrastructure links (backhaul, safety systems) should target 30 dB or more.

FSPL Reference Table

Free-space path loss for common frequencies and distances. Values in dB.

Frequency	Application	10 m	100 m	1 km	10 km	100 km
433 MHz	LoRa, ISM, RC	45.2	65.2	85.2	105.2	125.2
868 MHz	LoRa EU, Zigbee	51.2	71.2	91.2	111.2	131.2
915 MHz	LoRa US, LTE Band 8	51.7	71.7	91.7	111.7	131.7
1575 MHz	GPS L1	56.4	76.4	96.4	116.4	136.4
2400 MHz	Wi-Fi, BT, Zigbee	60.0	80.0	100.0	120.0	140.0
3500 MHz	5G NR Sub-6	63.3	83.3	103.3	123.3	143.3
5800 MHz	Wi-Fi 5 GHz, P2P	67.7	87.7	107.7	127.7	147.7
11 GHz	Microwave backhaul	73.3	93.3	113.3	133.3	153.3
24 GHz	Radar, backhaul	80.1	100.1	120.1	140.1	160.1
60 GHz	WiGig, mmWave P2P	88.0	108.0	128.0	148.0	168.0

Typical Link Budgets for Common Systems

System	TX Power	TX Gain	RX Gain	RX Sensitivity	FSPL budget
LoRa SF12, 868 MHz	14 dBm	2 dBi	2 dBi	−137 dBm	155 dB
Wi-Fi 802.11n, 2.4 GHz	20 dBm	3 dBi	3 dBi	−70 dBm	96 dB
5G NR FR1, 3.5 GHz	23 dBm	15 dBi	0 dBi	−100 dBm	138 dB
P2P 5.8 GHz bridge	27 dBm	23 dBi	23 dBi	−75 dBm	148 dB
Microwave backhaul 11 GHz	27 dBm	34 dBi	34 dBi	−80 dBm	175 dB

EIRP regulations: Maximum EIRP is regulated in most jurisdictions. In the EU, 2.4 GHz Wi-Fi is limited to 20 dBm EIRP indoors and 100 mW (20 dBm) outdoors. The US FCC Part 15 allows up to 36 dBm EIRP for point-to-point 5.8 GHz links with directional antennas. Always verify local regulations before deploying high-gain antennas.

Free Space Path Loss Calculator