Free Space Path Loss & Link Budget Calculator

Link budget and estimated range for LoRa, WiFi and BLE from TX power, gains and sensitivity.

Example: At 1 km and 868 MHz, path loss is 91.2 dB — leaving 63.8 dB of margin against a -137 dBm receiver.

Inputs

Frequency(Hz)

Distance(m)

Transmit power(dBm)

14 dBm is the EU868 legal limit; Bluetooth LE is typically 0 to 8.

TX antenna gain(dBi)

RX antenna gain(dBi)

Receiver sensitivity(dBm)

−137 for LoRa at SF12, around −95 for BLE, −90 for WiFi.

Result

Link margin

63.8 dB

Free space path loss: 91.2 dB
Received power: -73.2 dBm
Total link budget: 155 dB
Theoretical max range: 1.547 Mm
Realistic outdoor range: 464.1 km

63.8 dB of margin at 1 km. Anything above 20 dB is comfortable for a fixed outdoor link.
Free-space loss is the best case and never happens in practice. Real links lose 20–30 dB more indoors, and a single wall costs 5–15 dB depending on construction. Treat the theoretical range as an upper bound, and expect roughly a third of it outdoors with clear line of sight.
Path loss doubles in dB terms every time you double the distance — 6 dB per doubling. That also means 6 dB of extra budget only buys twice the range, which is why chasing distance through transmit power alone gets expensive fast.

Margin against distance

Distance	Path loss	Received	Margin
10 m	51.2 dB	-33.2 dBm	103.8 dB
100 m	71.2 dB	-53.2 dBm	83.8 dB
500 m	85.2 dB	-67.2 dBm	69.8 dB
1 km	91.2 dB	-73.2 dBm	63.8 dB
5 km	105.2 dB	-87.2 dBm	49.8 dB
10 km	111.2 dB	-93.2 dBm	43.8 dB

Formula

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

Worked example

At 1 km and 868 MHz, path loss is 91.2 dB — leaving 63.8 dB of margin against a -137 dBm receiver.

FSPL = 20·log₁₀(d_km) + 20·log₁₀(f_MHz) + 32.44
20·log₁₀(1) + 20·log₁₀(868) + 32.44
91.21 dB
received = TX + gains − loss
14 + 2 + 2 − 91.2
-73.2 dBm
margin = received − sensitivity
-73.2 − (-137)
63.8 dB

Frequently asked questions

How far will my radio link reach?

Free-space calculation gives an upper bound that real links never achieve. Expect roughly a third of it outdoors with clear line of sight, and far less indoors — a single interior wall costs 5–15 dB, which alone can halve the range.

How much link margin do I need?

At least 10 dB, and 20 dB for anything that must be reliable. Margin absorbs fading, rain, antenna misorientation and the difference between a datasheet's sensitivity figure and your actual board. A link with 3 dB margin works on the bench and fails in the field.

Why does doubling transmit power barely help?

Because path loss rises 6 dB per doubling of distance, and doubling power only gives 3 dB. You need four times the power to double the range in free space — and worse than that in the real world. Antenna gain and a better receiver are usually cheaper.

Does a lower frequency really travel further?

For the same power, yes — path loss rises 6 dB per doubling of frequency, so 433 MHz reaches roughly twice as far as 868 MHz. Lower frequencies also penetrate obstacles better. The trade-off is a physically larger antenna and less available bandwidth.

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