What is RSSI? How a Bluetooth tracker estimates distance

RSSI stands for Received Signal Strength Indicator: a number describing how loud a radio packet was when it arrived, expressed in decibel milliwatts and always negative in practice. If you have ever wondered what is RSSI doing inside a finding tag, the answer is that it is the only distance clue a plain Bluetooth device has — and it is a clue rather than a measurement. This piece defines the term, sets out the maths that turns it into a distance guess, and explains why that guess is often wrong.

What is RSSI, exactly?

It is the receiver's report of arriving signal power, not a property of the transmitter and not a distance.

A Bluetooth radio that receives a packet can measure how much power arrived at its antenna. That value is reported as RSSI, typically between about −30 dBm for a device pressed against the phone and about −100 dBm at the edge of usable reception. Because it is a decibel figure relative to one milliwatt, and the power involved is a tiny fraction of a milliwatt, the number is negative: −45 dBm is a strong signal and −92 dBm is a weak one.

Two points follow immediately. Higher numbers are closer to zero, so −50 is stronger than −80. And the scale is logarithmic, so the gap between −50 and −60 represents a tenfold drop in power, not a modest one.

How does a tracker turn RSSI into a distance?

With a path-loss model: an equation relating signal strength to distance, calibrated with a reference reading at one metre.

In free space, radio power spreads over the surface of an expanding sphere, so received power falls with the square of distance. In decibels that produces the rule of thumb worth remembering: about 6 dB of loss every time the distance doubles.

The estimate is made by comparing the measured RSSI against a known value at a reference distance — often one metre — and solving for distance using an exponent that describes how lossy the environment is. In an empty field that exponent is close to the free-space value of 2. In a furnished house it can be 3 or 4, and nothing in the packet tells the phone which it is dealing with.

That is the fundamental weakness. The equation is exact; the environment it assumes is a fiction.

Which four things make RSSI lie?

Absorption, reflection, orientation and interference — each capable of shifting the reading by more than a real change of position would.

1. Absorption. The 2.4 GHz band is absorbed by water, and a human body is largely water. Turning your back on a tag can cost as much apparent distance as walking several metres away from it.
2. Reflection and multipath. Signals bounce off radiators, appliances, metal door frames and wet walls, arriving by several routes at once. The sum can be stronger or weaker than the direct path alone, and it changes as anybody in the room moves.
3. Antenna orientation. A small antenna does not radiate equally in all directions. A tag lying flat and the same tag hanging vertically produce different readings from the identical spot.
4. Interference. Wi-Fi, microwave ovens, wireless doorbells and baby monitors share the band. A noisy neighbour raises the floor against which your packet has to be heard.

How accurate is an RSSI distance estimate in practice?

Good enough to say near, nearer or further away, and not good enough to say four metres.

Used carefully — by walking, watching the trend, and treating a rising signal as "warmer" — RSSI is genuinely useful. Used as a literal readout it is misleading, which is why responsible apps show a proximity band or a "with you" state rather than a number in metres.

The industry's answer to this limitation was to stop measuring loudness and start measuring time. Ultra Wideband times the flight of a pulse between two radios, and the Bluetooth SIG's own Channel Sounding, introduced in Core Specification 6.0, adopted on 3 September 2024, combines phase-based ranging with round-trip timing. Both give a true distance. Both need a second radio or new silicon at each end, which is why they appear in premium devices first. Our explanation of how accurate tags really are covers what that means for everyday searching.

What does RSSI mean for the tag you own?

It sets the outer limit of a live connection and decides nothing else you will notice.

When a tag is in range, the practical questions are whether your phone can reach it and whether you can hear the speaker. RSSI governs the first. Once you are close enough to ring it, sound locates the object far better than any signal-strength curve, because your ears do genuine direction-finding and a decibel reading does not.

When a tag is out of range, RSSI is irrelevant entirely. The position then comes from a passing device's own location fix, which is a different kind of measurement with a different kind of error.

How BlueTag uses signal strength

BlueTag is a plain Bluetooth Low Energy broadcaster, so signal strength is exactly the mechanism described above: your iPhone hears the advertisement, judges from its strength whether the tag is nearby, and shows the item as being with you or not. The usable link runs to about 60 m / 200 ft in the open and considerably less through a brick wall, a rucksack or a steel case.

It carries no Ultra Wideband and no Channel Sounding hardware, so there is no arrow and no metre reading — we would rather set that expectation here than have a review set it for us. The last few metres are closed by the 80–100 dB buzzer, and everything past the Bluetooth link is handled by the Apple Find My network, with each sighting encrypted to your Apple Account alone.

The rest of the specification is fixed and small: a 36.3 × 36.3 × 9 mm case weighing around 10 g, sealed to IP67, with a replaceable CR2032 rated at a couple of years. It needs an iPhone or an iPad from iOS 14.5 upwards and does not work with Android in any form. Two per box, delivered free in the UK, under £15. See the specification.