Estimating beacon proximity/distance based on RSSI - Bluetooth LE

The txPower mentioned by @davidgyoung is given by the formula:

RSSI = -10 n log d + A

where

• d = distance
• A = txPower
• n = signal propagation constant
• RSSI = dBm

In free space n = 2, but it will vary based on local geometry – for example, a wall will reduce RSSI by ~3dBm and will affect n accordingly.

If you want the highest possible accuracy, it may be worthwhile to experimentally determine these values for your particular system.

Reference: see the paper Evaluation of the Reliability of RSSI for Indoor Localization by Qian Dong and Waltenegus Dargie for a more detailed explanation of the derivation and calibration.

double getDistance(int rssi, int txPower) {
    /*
     * RSSI = TxPower - 10 * n * lg(d)
     * n = 2 (in free space)
     * 
     * d = 10 ^ ((TxPower - RSSI) / (10 * n))
     */

    return Math.pow(10d, ((double) txPower - rssi) / (10 * 2));
}