Calculate free-space path loss, estimated receiver power, and the margin your RF link has above its required target.
Transmitter
Path
Receiver
Advanced options
Installed losses
Planning target and display
Table of contents
How to use our Free-Space Path Loss and RF Link Budget Calculator
- Choose how to find path loss: enter operating frequency and straight-line antenna distance, or select a supplied path-loss value.
- Enter conducted TX output power and its unit, then enter TX antenna gain. Do not use EIRP as TX output power.
- Enter RX antenna gain and Receiver sensitivity for the exact planned radio mode, bandwidth, data rate, modulation, and coding.
- Open Advanced options to add installed losses, extra path loss, or a Required design margin if your project has documented values.
- Click Calculate, then check that Estimated receiver-input power is above Receiver sensitivity and that Margin above or below the required target is positive. Treat the planning status as a first-pass check, not proof of a working field link.
Definitions
dB: A relative unit for a gain or loss. Antenna gain and cable loss are commonly stated in dB.
dBm: An absolute power level referenced to 1 milliwatt. Negative dBm values are normal for received radio signals; a less negative value is stronger.
dBi: Antenna gain in dB compared with an ideal isotropic antenna, which radiates equally in all directions.
Free-space path loss (FSPL): Signal spreading loss over an unobstructed straight-line path. It rises as frequency or distance rises.
EIRP: Effective isotropic radiated power. It is conducted TX power plus TX antenna gain, minus entered TX-side losses.
Receiver sensitivity: The minimum receiver-input power needed for a specific radio mode. It must match the planned bandwidth, data rate, modulation, and coding.
Available link margin: Estimated receiver-input power minus Receiver sensitivity. A positive value is above the sensitivity threshold.
Common mistakes and quick fixes
Mistake: Entering EIRP in TX output power and also entering TX antenna gain.
Fix: Enter conducted radio power in TX output power. The calculator creates Estimated EIRP after it adds TX antenna gain and subtracts TX-side losses.
Mistake: Using a road route, cable run, or walking route for Straight-line antenna distance.
Fix: Measure the direct antenna-to-antenna separation and choose the matching Distance unit.
Mistake: Copying 915 MHz as 915 GHz, or changing Frequency unit without changing the value.
Fix: Copy the number and its printed unit together into Operating frequency and Frequency unit.
Mistake: Using a Receiver sensitivity number for a different data rate or bandwidth.
Fix: Find the sensitivity row for the planned receiver mode, bandwidth, modulation, and coding before entering Receiver sensitivity.
Mistake: Putting a desired safety buffer into Extra path loss.
Fix: Check How should path loss be found? and then recalculate. Put a documented physical penalty in Extra path loss and put the project target in Required design margin.
Mistake: Treating a positive Free-space planning status as a guarantee.
Fix: Check the site for obstructions, interference, multipath, foliage, weather effects, and antenna polarization before relying on the estimate.
Limitations & Key Assumptions / Boundary Conditions
- This is a free-space, straight-line model. It does not predict loss from buildings, terrain, trees, people, Fresnel-zone blockage, or indoor materials.
- It does not model interference, receiver desensitization, multipath fading, polarization mismatch, weather, or changing antenna alignment.
- Use antenna gain and feedline loss values at the actual operating frequency. Catalog peak gain or loss at another band can give a wrong budget.
- Receiver sensitivity depends on radio settings. A sensitivity value from a different bandwidth, data rate, modulation, or coding setting can make the margin misleading.
- Extra path loss and Required design margin are user assumptions. Blank optional loss fields and a blank target margin count as 0 dB.
- A nonnegative Margin above or below the required target only means the entered free-space budget meets that entered target. Site survey, field testing, applicable power rules, and antenna safety requirements remain separate checks.
Methodology
Calculation method
The calculator converts TX output power to dBm, operating frequency to hertz, and straight-line antenna distance to meters. The speed of light used is exactly 299,792,458 m/s. [2]
FSPL (dB) = 20 log10(4 pi d f / c)
In this formula, d is distance in meters, f is frequency in hertz, and c is the speed of light in meters per second. If you select Enter known FSPL, the supplied Known free-space path loss replaces this calculation.
Estimated EIRP (dBm) = TX power (dBm) + TX antenna gain (dBi) - TX-side losses (dB)
Estimated receiver-input power (dBm) = TX power + TX antenna gain + RX antenna gain - all entered losses
Available link margin (dB) = Estimated receiver-input power - Receiver sensitivity
Margin above or below the required target (dB) = Available link margin - Required design margin
This gain-and-loss accounting follows the usual RF link-budget structure: gains are added, and loss terms are subtracted. [3]
Worked example
For 30 dBm TX output power, 10 dBi TX gain, 10 dBi RX gain, 1 GHz, and 1 km, FSPL is about 92.45 dB. With no extra losses, estimated receiver-input power is about -42.45 dBm. With Receiver sensitivity of -90 dBm, available link margin is about 47.55 dB. If Required design margin is 10 dB, the final target gap is about +37.55 dB.