Distance Estimation from RSSI
Edit on GitHubTurning an RSSI reading into a rough distance estimate, why it is rough, and the calibration walk that makes it honest.
Distance Estimation from RSSI
“RSSI -90, so how far?” is the most asked question in beacon work, and the honest answer is: it depends. This page turns the dependence into a usable estimate.
The free-space reference
In ideal open terrain, the path loss at 433 MHz follows the inverse-square law:
loss (dB) = 32.4 + 20 x log10(distance_m) + 20 x log10(433)At 1 km the loss is about 85 dB. A beacon at +17 dBm then arrives at about -68 dBm. The catch: real terrain adds 10 - 40 dB on top of free space (see Foliage Loss, Mountain Effects on Radio).
The honest table (open terrain)
| RSSI (dBm) | Distance |
|---|---|
| -60 | ~100 - 200 m |
| -70 | ~400 - 800 m |
| -80 | ~1 - 2 km |
| -90 | ~2 - 5 km |
| -100 | 5 km+, or terrain-limited |
The terrain multiplier
In forest or valleys, divide the distance by 3 - 10. A -80 dBm reading in open country means kilometers; the same reading in dense forest means hundreds of meters. The terrain is the dominant term.
The calibration walk
The honest tool is a calibration walk, once per area:
- Walk away from the beacon in 100 m steps.
- Record RSSI at each step (see Signal Strength Mapping).
- Build the area’s personal distance-RSSI table.
That table beats any formula for that area. The Beacon Log Template records the calibration.
Related pages
- RSSI Explained for the units
- RF and Link Budget for the math
- Decibel Table for the mental arithmetic