Link Budget Calculator: Estimate a Radio Link

A link budget calculator estimates the received signal power in a radio link by adding transmitter power and gains, then subtracting cable, propagation, and other losses. Enter the relevant values in consistent units to see the estimated received power and the link margin above a receiver’s sensitivity; these results help assess a link, but do not guarantee real-world coverage.

Formula

The calculator uses a basic decibel link-budget equation:

Received power, P_r (dBm) = transmit power, P_t (dBm) + transmit antenna gain, G_t (dBi) + receive antenna gain, G_r (dBi) − transmit-side losses, L_t (dB) − path loss, L_p (dB) − receive-side losses, L_r (dB).

Link margin (dB) = received power (dBm) − receiver sensitivity (dBm). All power terms use dBm, antenna gains use dBi, and losses and margin use dB. The input groups for side losses can include relevant feed-line, connector, filter, or implementation losses, but avoid counting any loss twice. Path loss represents attenuation between the antennas; it may be estimated from a propagation model or measured for the environment.

Worked Example

Using the calculator’s defaults: transmit power is 20 dBm, each antenna gain is 2 dBi, transmit-side and receive-side losses are 1 dB each, path loss is 100 dB, and receiver sensitivity is −100 dBm.

• Add transmitter power and antenna gains: 20 + 2 + 2 = 24 dBm.
• Subtract all listed losses: 24 − 1 − 100 − 1 = −78 dBm received power.
• Compare received power with sensitivity: −78 − (−100) = 22 dB link margin.

The displayed results are therefore −78.0 dBm received power and 22.0 dB link margin. This arithmetic assumes that every entry describes the same operating frequency, channel, and link direction.

What a Link Budget Tells You

A link budget is an accounting of signal power from a transmitter to a receiver. It is useful in wireless network planning, point-to-point radio links, satellite communications, and other radio systems. Rather than treating a signal as simply “strong” or “weak,” the calculation identifies the contributions that raise or reduce the power available at the receiver.

The received-power result is an estimate at the receiver input, under the assumptions used for the gains and losses. The margin compares that estimate with the receiver’s sensitivity: a positive number means the estimated signal is above the stated sensitivity threshold; zero means it is exactly at that threshold; a negative number means it falls below it. Sensitivity is not a universal value for a receiver. It depends on equipment and operating conditions, including modulation, data rate, bandwidth, and the performance criterion used to specify sensitivity.

A positive margin is not a promise of reliable service. Designers may account for fading, interference, obstruction, weather, polarization mismatch, and changing equipment or installation conditions. Those effects can vary over time and location, and may require separate estimates or measurements.

How to Choose Inputs

Use figures from equipment specifications, cable and component data, field measurements, or an appropriate propagation calculation. Check that the values apply to the intended frequency and configuration.

• Transmit power: Use the power delivered to the transmit-side chain at the point represented by the budget. Confirm whether a specification is in dBm, watts, or another unit before converting.
• Antenna gain: Enter gain in dBi, which references an ideal isotropic radiator. A value in dBd uses a different reference and must be converted before combining it with dBm and dBi.
• Losses: Include cable, connector, duplexer, filter, or other relevant losses in the appropriate side-loss entry. Use total loss in dB, not a positive gain.
• Path loss: Obtain a value that reflects the frequency, distance, antenna heights, and environment. Free-space path loss is a baseline for an unobstructed idealized path, not a full prediction of a cluttered or indoor route.
• Receiver sensitivity: Use the specification for the required mode and performance threshold, rather than a number for a different data rate or bandwidth.

Units and Common Pitfalls

Decibels are logarithmic. In a dBm power value, 0 dBm equals 1 milliwatt; a change of 10 dB corresponds to a tenfold power ratio, and 3 dB is approximately a twofold ratio. Gains and losses in dB can be added or subtracted in a budget, but absolute power in watts should not be added directly to a dBm value.

Pay particular attention to signs and reference points. Antenna gain is added; attenuation is subtracted. A loss entered as a negative number and then subtracted would mistakenly increase the calculated signal. Also, do not include antenna gain or cable loss in more than one input. If transmit power is already specified as EIRP (effective isotropic radiated power), it already includes transmitter-side antenna gain and relevant losses up to the reference point; do not add those items again. EIRP is a transmit-side quantity, not the received power.

Limits and Useful Comparisons

This simple calculator does not model interference, receiver noise, fading distributions, terrain, diffraction, reflections, or antenna patterns. It also does not determine throughput, coverage area, regulatory compliance, or whether a system will maintain a particular connection quality. A more detailed design may require a propagation model, link measurements, and separate analysis of noise and interference.

For free-space path loss, a commonly used form is FSPL (dB) = 32.44 + 20 log₁₀(distance in km) + 20 log₁₀(frequency in MHz). This formula is only a free-space estimate and its constant assumes those particular units; inserting meters or GHz without adjusting the constant produces an incorrect result. The path-loss input here is manual so a reader can use an estimate appropriate to the link rather than treating free-space loss as universally valid.

Frequently Asked Questions

What is a link budget calculator?
A link budget calculator adds transmitter and antenna gains and subtracts losses to estimate received radio power. It can also compare that estimate with receiver sensitivity to show a basic link margin.

What does a positive link margin mean?
It means the estimated received power is above the sensitivity value entered. The size of the margin does not by itself establish reliability, because fading, interference, and other conditions may not be included.

Is dBm the same as dB?
No. dBm expresses absolute power relative to 1 milliwatt. dB expresses a ratio, such as a gain or loss. A budget combines dBm power with gains and losses in dB.

Should I enter EIRP as transmit power?
You can use EIRP as the transmit-side starting point, but then do not add antenna gain or losses already included in that EIRP figure. Keep the budget’s reference points consistent.

Does a positive margin guarantee a working radio link?
No. It is a calculation based on entered values and assumptions, not a guarantee. Interference, obstruction, changing conditions, receiver configuration, and other factors can affect actual performance.

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in tx | Transmit power | dBm | 20 | -100 | 100
in tg | Transmit antenna gain | dBi | 2 | -100 | 100
in rg | Receive antenna gain | dBi | 2 | -100 | 100
in tl | Transmit-side losses | dB | 1 | 0 | 200
in pl | Path loss | dB | 100 | 0 | 300
in rl | Receive-side losses | dB | 1 | 0 | 200
in sen | Receiver sensitivity | dBm | -100 | -200 | 50
out rx | Received power | dBm | tx + tg + rg - tl - pl - rl | 1
out margin | Link margin | dB | rx - sen | 1