What is a link budget?
A link budget tracks the power of a radio signal from a transmitter to a receiver. Transmit power and antenna gains increase the available signal level, while feeder losses and propagation losses reduce it. The result is an estimate of received power in dBm.
Compare received power with the receiver sensitivity for your selected operating mode. Their difference is the link margin. If the available margin is smaller than your required fade reserve, the design does not meet that reserve even when the signal is above sensitivity.
How to use this RF link budget calculator
- Enter the frequency and distance, or supply a modeled path loss.
- Enter transmitter output power, antenna gains and feeder losses.
- Add atmospheric, polarization and other losses that are not already included in the path loss.
- Estimate sensitivity using noise bandwidth, system noise figure and required SNR, or enter a datasheet sensitivity.
- Choose a fade margin target and inspect received power, available margin and remaining headroom. Download the inputs and results as CSV for your planning notes.
Use matching reference points throughout. A receiver sensitivity specified at an antenna connector must be compared with signal power at that same connector. Do not mix a downstream LNA output level with input-referred sensitivity.
RF link budget formulas
| Parameter | Description |
|---|---|
| PRX | Received Power (dBm) |
| PTX | Transmitter Output Power (dBm) |
| GTX | Transmitter Antenna Gain (dBi) |
| LTX | Transmitter Cable/Connector Loss (dB) |
| LFS | Free-Space Path Loss (dB) |
| LM | Additional losses (polarization, rain and other path losses; excludes fade margin target) |
| GRX | Receiver Antenna Gain (dBi) |
| LRX | Receiver Cable/Connector Loss (dB) |
1. EIRP
Transmitter output power is in dBm, antenna gain is in dBi and feeder loss is in dB. The resulting equivalent isotropically radiated power is in dBm.
2. Free-space path loss
The calculator uses the rounded 32.45 constant with frequency in MHz and distance in km. This is a far-field, unobstructed free-space model. Consult ITU-R P.525: Calculation of free-space attenuation for the propagation basis.
3. Received power at receiver input
Extra losses are the sum of rain/atmospheric, polarization and miscellaneous losses. The receiver input is after the receive feeder and before the optional LNA.
4. Noise floor and sensitivity
Sensitivity = N + SNRrequired
The −174 dBm/Hz approximation assumes thermal noise near 290 K. Bandwidth is the relevant noise bandwidth in Hz; NF is the complete receiver-chain noise figure referred to the selected receiver input. Datasheet sensitivity may be preferable when the operating mode and error-rate criteria are known.
5. Margin and remaining headroom
Headroom = Link margin − Fade margin target
Headroom of zero or more meets the entered reserve in this model. Fade reserve is a design requirement, not an additional loss to subtract twice.
Worked example: 3.5 GHz link over 5 km
Use 33 dBm TX power, 18 dBi TX gain, 21 dBi RX gain, 1.5 dB TX feeder loss and 1.2 dB RX feeder loss. Add 0.8 dB atmospheric loss and 0.5 dB polarization loss. Set system NF to 4.5 dB, bandwidth to 20 MHz, required SNR to 12 dB and fade reserve to 15 dB.
| Quantity | Calculation | Result |
|---|---|---|
| EIRP | 33 + 18 − 1.5 | 49.50 dBm |
| Free-space loss | 32.45 + 20 log₁₀(3500) + 20 log₁₀(5) | 117.31 dB |
| Received power | 49.50 − 117.31 − 1.30 + 21 − 1.20 | −49.31 dBm |
| Noise floor | −174 + 10 log₁₀(20,000,000) + 4.5 | −96.49 dBm |
| Sensitivity | −96.49 + 12 | −84.49 dBm |
| Link margin | −49.31 − (−84.49) | 35.18 dB |
| Headroom after reserve | 35.18 − 15 | 20.18 dB |
The example meets its 15 dB reserve under the entered assumptions. The optional 2 dB LNA gain gives an output signal level of −47.31 dBm; it is not added to the input-referred margin.
Using link budgets for LTE, 5G, Wi-Fi and microwave
The power accounting is useful across radio systems, but the assumptions must match the technology and operating mode. For LTE and 5G, keep transmit power, resource allocation, bandwidth and receiver sensitivity consistent. Total wideband received power is not automatically RSRP, and this thermal-noise calculation does not predict SINR in an interference-limited network.
For Wi-Fi, use sensitivity for the selected channel width and modulation/coding rate. For microwave links, consider rain attenuation, antenna alignment, Fresnel clearance and the required availability. For cellular or obstructed routes, enter a suitable modeled path loss instead of relying on free-space loss alone.
Common mistakes to avoid
- Entering watts in a dBm field: first use the watt to dBm converter.
- Using antenna gain in dBd as if it were dBi: add approximately 2.15 dB to convert dBd to dBi.
- Counting feeder or atmospheric losses twice when a model already includes them.
- Adding LNA gain directly to a margin calculated from input-referred sensitivity.
- Treating a positive margin as proof of coverage or a guaranteed uptime percentage.
FAQ on Link Budget Calculator
An RF link budget adds transmitter power and antenna gains, then subtracts cable and propagation losses to estimate received power. Comparing that power with receiver sensitivity gives the available link margin.
Link margin is received power minus receiver sensitivity. The fade margin target is the reserve you choose for changing conditions. Headroom is link margin minus that target. Do not also enter the same fade reserve as a propagation loss.
Yes, for a simplified power budget. Use consistent signal power, bandwidth and sensitivity for the same channel and operating mode. Cellular coverage planning also needs appropriate propagation, interference, antenna, resource allocation and reliability assumptions; this tool does not calculate RSRP or SINR.
Not automatically. An LNA amplifies signal and noise. It can improve the receiver chain noise figure, depending on its position and the other stages. Here, optional LNA gain changes only the displayed output signal power; enter the complete chain noise figure referred to the receiver input for margin calculations.
There is no single correct value for every link. Choose the reserve from the frequency, terrain, climate, propagation model and availability requirement. The 15 dB example is illustrative and is not an availability guarantee.
Thermal noise power increases with bandwidth. With noise figure and required SNR unchanged, doubling bandwidth raises the noise floor and required received power by approximately 3.01 dB.
No. Free-space path loss represents ideal unobstructed propagation. Add suitable extra losses or enter a path loss from a more appropriate model. Avoid counting losses again if they are already included in your entered path loss.