What dBm actually means on a signal meter

Signal bars are a marketing decision. There is no standard that says how many bars correspond to how much received power, so the same spot in the same room can show four bars on one handset and two on another sitting beside it. The number underneath is not a matter of opinion: dBm is an absolute unit of received power, defined the same way by every chipset in every network. Once you can read it, the bars become what they always were — a rough summary drawn by the phone's manufacturer — and you get to work with the measurement itself.

A logarithmic unit, and why it is negative

The unit means decibels relative to one milliwatt. Zero dBm is exactly one milliwatt of received power. Because radio power arriving at a handset is a minuscule fraction of that, every value you will ever see indoors is negative, and the closer to zero the stronger the signal. Minus eighty is far better than minus one hundred, which trips up almost everyone at first. The scale is logarithmic, which is what makes it usable: an increase of three decibels means double the power, ten decibels means ten times the power, and twenty decibels means a hundredfold. A move from minus one hundred and ten to minus eighty is not a thirty per cent improvement, it is a thousand times more energy landing on the antenna.

The range that matters in practice

A phone standing under a mast can see minus fifty or better. The floor of usefulness sits somewhere near minus one hundred and twenty, where the wanted signal disappears into thermal noise and the radio can no longer decode anything reliably. Everything interesting happens in the seventy decibels between those extremes, and the practical thresholds are surprisingly sharp: voice survives on very little, while high throughput needs both a strong signal and a clean one. That is why a value near minus one hundred often still carries a call perfectly but makes a video stall — the modem drops to a slower, more robust coding scheme long before the connection fails outright.

Reading the bands

The table below gives the working thresholds for reference signal received power on a modern network. Treat the boundaries as soft: a value sitting on a line will behave like the better category on an empty cell and like the worse one at eight in the evening in a crowded street. One more caveat belongs with the table: the same figure means different things on different frequencies. A reading of minus one hundred on a low band that has already fought its way through two concrete walls is a healthier situation than the same reading on a high band with a clear view of the mast, because the low band still has margin left for the next obstacle while the high band does not. Read the number together with the frequency whenever you have both.

Why two phones disagree

Three effects explain almost every disagreement. Antenna design differs between models, and a couple of decibels of gain in the same casing position is entirely normal. Reporting differs too: some chipsets average over a longer window, so one phone reacts to a step behind a wall in a second while another takes five. And the bars mapping is unique to each manufacturer, so the same minus ninety-eight can be drawn as two bars or three. Comparing raw dBm between handsets held in the same hand, at the same height, facing the same way, is the only comparison worth making.

The other numbers on the same screen

Older networks report a plain wideband power figure, so on third and second generation service you will see a received strength that includes everything in the channel. Fourth and fifth generation service reports reference power instead, measured only on the known pilot symbols, which is a cleaner measurement and typically reads about twenty decibels lower than the wideband figure for the same conditions. That offset is the single most common source of confusion when someone compares a screenshot of a 3G reading with a 4G one. There is also a small integer called the arbitrary strength unit, kept for compatibility, which on modern networks is simply the reference power plus one hundred and forty.

Turning the number into a decision

The practical use of an absolute number is comparison across places and times. Stand where the call dropped and note the value. Walk to the window and note it again. If the difference is ten decibels or more, the fix is furniture, not the operator. If the whole flat reads within a few decibels of the same poor value, no amount of moving will help and the answer is Wi-Fi calling or a femtocell. If the reading is strong but the connection is still slow, the problem is not strength at all but quality and load, which are measured by different fields entirely.