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What dBm Means for Phone Signal Strength

Bars are easy to glance at and hard to compare. dBm is the measurement behind many of those icons. It records how much radio power the phone is receiving, on a scale that engineers use because the raw wattage of a distant signal is a very small number. Once you can read the sign and the direction, phone signal strength stops being a cartoon.

Why the number is negative

dBm compares power to one milliwatt. A handset almost never receives anything close to a milliwatt from a distant site, so the reading is negative. −70 dBm is a small fraction of a milliwatt. −100 dBm is a much smaller fraction. Closer to zero means stronger. A reading that moves from −80 dBm to −95 dBm got worse, even though 95 looks like a bigger number if you forget the minus sign.

The scale is logarithmic. A change of a few dBm is a real change in power, not a rounding error. That is why a short walk indoors can matter. You may only see one bar move, while the underlying reading moved enough to shrink the margin the radio had against noise and walls.

A field scale, not a law

A practical grouping used on this site runs roughly like this. Excellent is about −50 to −70 dBm. Good is about −70 to −85 dBm. Fair is about −85 to −100 dBm. Weak is about −100 to −110 dBm. Very weak sits below about −110 dBm. These are guide rails for reading a number. They are not a rule that every phone and every network obeys.

Device radios differ. Antennas differ. A phone in a case, or held so the antenna is covered, can report a softer number than the same phone held differently in the same spot. Networks also decide how they present signal in their own software. Two handsets side by side can disagree by several dBm and still both be honest about what their own radios measured.

What dBm does not include

The figure is received power. It is not latency, and it is not a count of how many people are using the site. A strong dBm reading can sit next to slow pages if the site is busy. A modest reading can still carry a clear call if the link is steady and the demand is light. Quality measures, such as how clean the signal is relative to noise and interference, sit beside power. A phone can see a mid-range number that is clean, or a similar number that is messy because of reflections inside a building.

Bars hide that detail by design. Makers pick thresholds and draw an icon. One model may still show several bars at −100 dBm. Another may have dropped to a single bar. Comparing bars across brands is a weak way to compare phone signal strength. Comparing dBm on one phone, in one grip, at two places, is stronger.

How to use a reading

If your phone shows a dBm value in its ordinary status or field screen, write it down outdoors, by a window, and in an interior room. Note the time. A drop of more than a handful of dBm as you walk in is the building spending the link. A number that stays strong while pages crawl is a clue to look at congestion and delay, not at the icon.

Do not chase a single perfect threshold. The useful question is the difference between spots and times on the same device. If −88 dBm at the window becomes −108 dBm at the back desk, you have learned where the loss is. If both spots read about −90 dBm and only the evening feels slow, the power did not change. The shared site did.

A short conclusion

dBm is a negative power reading. Less negative means a stronger phone signal. Use the ranges as a notebook scale, and expect phones to disagree at the edges. Pair the number with a place and a time. Power explains the radio budget. It does not, by itself, explain why a page is waiting.