Bluetooth Signal Strength and Distance
Signal strength is the only real information a Bluetooth scan gives you, so it is worth knowing how to read. The short version: the number tells you roughly how far away something might be, and changes in the number tell you reliably whether you are getting warmer. Use the first to choose where to start and the second to actually find the thing.
The scale
Received signal strength — RSSI — is reported in decibel-milliwatts, written dBm. The values are negative, and closer to zero means stronger. It is a logarithmic scale, so a difference of 10 dBm is a tenfold difference in received power, not a small nudge.
| Reading | Roughly means | What to do |
|---|---|---|
| −30 to −45 dBm | Very close — a few feet, possibly the object in front of you | Sweep surfaces and cavities by hand; try to make it beep |
| −45 to −60 dBm | Close — same part of the room, or just behind something | Short moves, watch for a sharp jump near a specific object |
| −60 to −75 dBm | Same room, or the next one through a thin wall | Walk the room in a loop and find where it peaks |
| −75 to −90 dBm | Far, or nearby but heavily obstructed | Move slowly through the space; expect the reading to jump about |
| Below −90 dBm | Edge of detection; readings will come and go | Treat any reading at all as confirmation it has power |
Strength and distance
In free space, signal strength falls with the square of distance, which works out to about 6 dB of loss every time distance doubles. That gives a usable rule of thumb: −6 dB means roughly twice as far, and +6 dB means roughly half as far.
| Change from your baseline | Approximate change in distance, open air |
|---|---|
| +12 dB stronger | About a quarter of the distance |
| +6 dB stronger | About half |
| No change | Same distance — or you moved along an arc around it |
| −6 dB weaker | About twice as far |
| −12 dB weaker | About four times as far |
Indoors, none of this holds precisely, because obstacles absorb signal in ways distance alone does not explain. That is not a reason to ignore the number — it is a reason to treat it as a radius rather than a point.
What each obstacle costs you
Bluetooth operates at 2.4 GHz, a frequency that water absorbs particularly well. This is the single most useful fact for searching, because it explains why devices hidden in soft furnishings read so much further away than they are.
| Obstacle | Approximate loss | Feels like |
|---|---|---|
| Your own body between phone and device | 3 to 10 dB | Up to three times further away |
| Sofa cushion or mattress | 3 to 8 dB | Twice as far or more |
| Plasterboard wall or wooden door | 3 to 6 dB | About twice as far |
| Brick or concrete wall | 10 to 20 dB | Four to ten times further |
| Tiled bathroom or kitchen wall | 10 to 25 dB | Often reads as no signal at all |
| Metal drawer, appliance, car body | 20 dB to total blockage | Undetectable, even from a few feet |
| Floor between storeys | 15 to 25 dB | Usually the end of detection |
Three things that will mislead you
Multipath fading
Radio reflects off walls, floors and appliances. Reflected copies can arrive out of phase with the direct signal and partially cancel it, creating dead spots a few inches wide. So the signal can weaken as you walk toward the device. If a reading contradicts your expectation, step sideways a foot and read again before reversing course.
Your hand and your phone's orientation
Phone antennas are not omnidirectional, and your hand absorbs signal. Rotating the phone can change a reading by several decibels without you moving at all. Hold it consistently — flat in an open palm, screen up — so that changes you see are changes in position, not grip.
Advertising interval
A dormant device may advertise only once every couple of seconds, so the display can look frozen or drop out entirely between packets. That is not a weak signal, it is an infrequent one. Give it time before drawing conclusions, especially for something lost hours ago.
Using this to search
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Baseline for ten seconds
Learn what the reading does when you are not moving, so you can recognise real change.
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Convert the number into a radius, not a point
A −70 dBm reading on an earbud means “somewhere in this part of the room, probably nearer than it looks”. Start there.
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Move in fixed steps and compare
Ten feet at a time, slowly. Note the reading at each stop. Look for the peak.
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Halve your steps as it strengthens, and sweep low
Sofa seams, bag linings, under furniture. Watch for a sharp rise as you approach one specific object — that jump is more informative than any absolute value.
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Finish with sound
Connect and play audio. Ears beat radio for the last few feet, every time.
Find it with Radar
The app is not out yet. In the meantime, the walk-the-signal method in these guides is exactly the technique it automates — and it works with the phone you already have.
Related guides
- How finder apps work
Where RSSI comes from, and why one phone cannot show a direction arrow.
- Lost earbuds that are turned off
Putting the method to work on the hardest common case.
Frequently asked questions
What is a good Bluetooth signal strength in dBm?
Values are negative and closer to zero is stronger. Around −40 dBm means the device is very close, within a few feet. Around −65 dBm is a comfortable working signal across a room. Around −85 dBm is weak and near the edge of range, and below about −95 dBm the signal is usually too faint to be received reliably at all.
How do I convert RSSI to distance?
Only roughly, and only in open air. Signal strength falls by about 6 dB each time distance doubles, so if you know the strength at one foot you can estimate from there. In a real room, absorption from walls, furniture and bodies makes the result read further than the truth, often by a factor of two or three. Use it to pick a search radius, never as a measurement.
Why does the signal get weaker when I walk toward the device?
Usually multipath fading. Radio waves reflecting off walls and floors can arrive out of phase with the direct signal and partially cancel it, creating dead spots a few inches across. Step sideways a foot or two and take another reading rather than concluding you are heading the wrong way.
How much signal does a wall block?
Plasterboard or a wooden door typically costs a few decibels. A brick or concrete wall can cost 10 to 20 dB, which is like moving several times further away. Metal — appliances, filing cabinets, a car body — can block the signal almost entirely, which is why a device in a metal drawer may be undetectable from a few feet.
Why does my phone show a different signal strength than my friend’s?
Antenna design, phone orientation, case material and where your hand sits all change how well a phone receives. Absolute numbers are not comparable between phones. Comparisons on the same phone as you move around are what matter, and those are reliable.