What Is dBm? Understanding Mobile Signal Strength Readings

Anyone who has looked at the signal strength readings on their phone has probably seen a number like -85 dBm or -110 dBm and wondered what it actually means. dBm is the standard unit used across the telecommunications industry to measure mobile signal strength, and it's the number that determines whether your call will connect, your data will load, and your safety-critical apps will transmit when they need to.

Understanding dBm is the difference between guessing at coverage quality and making informed decisions about it. This article covers what dBm actually measures, what the numbers mean in practice, and why signal strength alone doesn't tell the whole story of mobile coverage.

What dBm actually measures

dBm stands for decibels relative to one milliwatt. It's a unit of measurement used to express the power level of a signal on a logarithmic scale, referenced against a fixed baseline of one milliwatt (1 mW) of power.

The logarithmic scale is what makes dBm useful for mobile signal measurement. Mobile signals cover an enormous range of power levels, from the strong signal right next to a cell tower to the faint signal that just barely reaches inside a warehouse. Expressing that range in raw watts would produce numbers with lots of decimal places and very little practical readability. The logarithmic dBm scale compresses that range into a set of readable numbers that engineers can compare and interpret quickly.

A few things to know about how the scale works:

• Zero dBm equals one milliwatt: This is the reference point. A signal measured at 0 dBm has a power level of exactly one milliwatt.
• Positive dBm values are stronger than one milliwatt: These are rarely seen in mobile signal measurements, since they represent power levels well above what a phone or wireless device would normally encounter.
• Negative dBm values are weaker than one milliwatt: Mobile signal measurements are almost always negative. Every mobile signal reading you'll see on a phone or in a signal report will be a negative number.
• A number closer to zero is a stronger signal: This is the counterintuitive part. A reading of -70 dBm is significantly stronger than -100 dBm, even though -100 looks like a bigger number. The negative sign matters.
• Every 3 dB change represents a doubling or halving of signal power: A signal at -70 dBm has twice the power of a signal at -73 dBm. A signal at -85 dBm has four times the power of a signal at -91 dBm. Small numeric changes on the dBm scale represent large changes in actual signal power.

That last point is why engineers pay attention to differences that look small. Moving from -95 dBm to -100 dBm sounds like a minor drop, but it represents a threefold reduction in signal power. In practice, that's the difference between a call that connects and one that doesn't.

What dBm readings mean in practice

The dBm scale can be broken down into practical bands that describe what different signal strengths actually feel like when using a mobile device.

-50 dBm to -70 dBm: Excellent signal

This is the range you'd expect right next to a cell tower or in a building with a well-designed in-building coverage (IBC) solution. Calls connect instantly, data speeds are at their maximum, and there's substantial headroom for the signal to degrade before problems appear. Most users will never see readings this strong in a typical commercial building.

-71 dBm to -85 dBm: Good signal

Strong, reliable coverage. Calls are clear, data is fast, and connections are stable. This is what a well-covered office, retail space, or residential area should look like. Safety-critical applications work as designed.

-86 dBm to -100 dBm: Fair signal

Usable, but starting to degrade. Calls will generally connect but may drop under load. Data speeds are slower, and connections are less stable. This is the band where users start to notice problems, particularly in high-demand areas like meeting rooms or crowded floors.

-101 dBm to -110 dBm: Poor signal

Coverage is unreliable. Calls drop frequently, data is slow or fails to load, and safety apps may fail to transmit. This is the range where most coverage complaints originate, and where an IBC solution starts to become necessary rather than optional.

Below -110 dBm: No usable signal

The signal is too weak to support reliable communication. Calls fail to connect, data doesn't work, and any dependency on mobile connectivity in these zones is unreliable. Below-ground areas, cold storage, and heavily shielded rooms often fall into this range.

These bands are useful benchmarks, but they're indicative rather than absolute. A signal at -90 dBm in a low-interference environment may perform better than a signal at -85 dBm in an environment with heavy interference or network congestion.

Why dBm alone doesn't tell the whole story

Signal strength in dBm is the most commonly referenced measurement, but it's not the only one that matters. A location can show good dBm readings and still deliver poor mobile performance, because signal strength is only one dimension of coverage quality.

Several other measurements work alongside dBm to give a complete picture.

Reference Signal Received Power (RSRP) is a more precise measurement of the signal strength from a specific cell, isolated from noise and interference. RSRP is measured in dBm and is the standard signal strength metric used in 4G and 5G network engineering. When someone talks about "signal strength" in a technical context, they usually mean RSRP.

Reference Signal Received Quality (RSRQ) measures how clean the signal is relative to interference. Two locations can have identical RSRP but very different RSRQ, and the one with worse RSRQ will drop calls and struggle to hold data sessions. RSRQ is measured in dB (not dBm) because it's a ratio between two power levels.

Signal to Interference plus Noise Ratio (SINR) measures how much of the signal is usable once interference and background noise are accounted for. Low SINR is one of the main reasons a location "has bars" but still doesn't work properly.

Received Signal Strength Indicator (RSSI) is the total power the device is receiving across the entire channel, including interference and noise. RSSI is a broader, less precise measurement than RSRP and is more useful as a general indicator than as an engineering metric.

A proper cellular assessment captures all of these measurements together. dBm gives the raw strength, but RSRQ and SINR reveal whether that strength is usable. A survey that only reports "bars" or a single dBm number is missing most of the picture.

Why phone signal bars are misleading

The signal bars displayed on a phone are an approximation, not a measurement. Different phone manufacturers and different operating system versions use different formulas to convert dBm readings into a bar count. Four bars on one phone might represent -85 dBm, while four bars on another phone might represent -95 dBm.

This is why two people standing in the same room with different phones will often see different bar counts, and why a device that "shows full bars" can still drop calls. The bars are a rough visual indicator for consumer reassurance, not a diagnostic tool.

For any serious assessment of coverage, the actual dBm reading is the number that matters. Most phones allow you to view the raw dBm value in a diagnostic menu, and professional survey equipment captures dBm continuously across every band and carrier.

How dBm readings inform IBC design

A cellular site survey uses dBm measurements as the foundation for every design decision that follows. The survey captures signal strength across the building, identifies zones where dBm readings fall below usable thresholds, and analyses why the signal is degrading in those zones.

From that data, an engineer can determine several things:

• Where the signal is strong enough to work with, and where it isn't
• How much amplification would be needed to bring dead zones up to usable levels
• Which carriers and bands are underperforming
• Whether a CEL-FI smart repeater is sufficient, or whether a Distributed Antenna System (DAS) is required
• Where antennas need to be placed to distribute signal effectively across the building

Without measured dBm data, none of these decisions can be made properly. An IBC solution specified without a survey is guesswork, and guesswork tends to produce systems that either underperform in the zones that matter or overspecify in zones that don't.

Measure the signal before making coverage decisions

dBm is the language of mobile signal strength, and understanding it is the first step to making informed decisions about coverage. But dBm alone is only part of the picture. Reliable coverage requires a signal that is strong, clean, and stable across every zone that matters, on every carrier the workforce uses.

MobileCorp designs and delivers in-building coverage (IBC) solutions built on measured, documented signal data. Every project begins with a cellular site survey that captures dBm readings, signal quality, and interference across the site, and produces a design recommendation grounded in what the data actually shows.

Request an Obligation-Free Site Assessment