How to use it
- Type the Frequency, MHz and the Distance, km between the two antennas.
- Type the Transmit power, dBm, Transmit antenna gain, dBi and Transmit cable and connector loss, dB, then the Receive antenna gain, dBi and Receive cable and connector loss, dB.
- Type the Receiver sensitivity, dBm from the receiver's datasheet.
- Read the Link margin, dB and the Verdict, then open Working, step by step to check the sums.
The free-space path loss
The free-space basic transmission loss is the loss between two isotropic antennas in free space. ITU-R P.525 gives it as 32.4 plus 20 times the logarithm of the frequency in MHz plus 20 times the logarithm of the distance in km. Each doubling of the distance adds about 6 dB, and so does each doubling of the frequency.
Lbf = 32.4 + 20 × log10(f) + 20 × log10(d)
Lbf free-space basic transmission loss, dB
f frequency, MHz
d distance, kmThe link budget and the margin
The received power adds the transmit power and the antenna gains, and takes away the cable losses and the path loss. The EIRP is the power that the transmit antenna sends towards the receiver. The link margin is the received power above the receiver sensitivity: a margin of zero or more means that the receiver can use the signal.
EIRP = Pt + Gt − Lt
Pr = Pt + Gt − Lt + Gr − Lr − Lbf
Margin = Pr − sensitivityWhat this calculator does not include
- Terrain, buildings and trees between the antennas, which add loss, and reflections from the ground, which can add or remove signal. ITU-R P.525 is the free-space case only.
- Rain and the atmosphere, which the free-space formula leaves out.
- Fading, interference and any safety margin that a design adds on top of the link margin.
Worked example: 2.4 GHz over 2 km
At 2400 MHz over 2 km the free-space path loss is 32.4 + 20 × log10(2400) + 20 × log10(2) = 32.4 + 67.60 + 6.02 = 106.02 dB.
With 20 dBm transmit power, a 12 dBi transmit antenna and 1 dB of transmit cable loss, the EIRP is 20 + 12 − 1 = 31.00 dBm. With a 2 dBi receive antenna and 1 dB of receive loss, the received power is 31 + 2 − 1 − 106.02 = -74.02 dBm.
With a receiver sensitivity of -90 dBm, the link margin is -74.02 − (-90) = 15.98 dB, so the link closes.
| Figure | Value |
|---|---|
| Free-space path loss | 106.02 dB |
| EIRP | 31.00 dBm |
| Received power | -74.02 dBm |
| Link margin | 15.98 dB |
Frequently asked questions
Why does the path loss rise by 6 dB when the distance doubles?
The loss grows with the logarithm of the distance: 20 log10 of 2 is about 6.02, so doubling the distance adds about 6 dB in free space. The power falls with the square of the distance, which is the same rule in decibels.
Why are the frequency in MHz and the distance in km?
Those are the units of the ITU-R P.525 equation (6), in which the constant 32.4 is written. Entering the same link in other units would need a different constant, so the calculator takes these two units only.
What is the difference between dBi and dBm?
dBi is a gain: the ratio of an antenna's power in its best direction to that of an isotropic antenna, in decibels. dBm is an absolute power, referred to one milliwatt, so 0 dBm is 1 mW.
Which sensitivity should I type?
Use the weakest input signal that the receiver's datasheet gives for the data rate or quality you need. A more negative sensitivity leaves a larger link margin for the same link.
Are the antenna heights and the terrain included?
No. Only the gains and the cable losses you type are included. The free-space value is the reference for the path, and a real path with obstacles or ground reflections can have more loss.
Are my numbers sent to a server?
No. The calculation runs in your browser, and nothing you type is stored or sent to us.
Sources
- ITU-R P.525-5, Calculation of free-space attenuation (11/2024), equations (5) and (6)
- Washington State University EE 432, lecture 6: Link Budgets (sensitivity and the received power sum)
Figures and text last checked on Thursday, November 12, 2026.