Signal strength is measured in decibels relative to one milliwatt, written as dBm. This scale runs from 0 dBm (strong) into negative numbers, where lower values like -90 or -100 dBm represent weaker signals. The dBm scale is logarithmic, not linear, which means a change of just 3 dB represents a doubling or halving of actual power. This makes it possible to express enormous differences in signal power using simple, manageable numbers.
Why Decibels Instead of Regular Numbers
Radio signals vary enormously in power. The signal hitting your phone’s antenna might be a billionth of a watt, while the signal leaving a cell tower could be several watts. Expressing that range in regular units would mean juggling numbers with many decimal places. The decibel scale compresses this into a practical range, typically between 0 and -120 dBm for most consumer wireless technologies.
The other advantage is simplicity. Because the scale is logarithmic, gains and losses in a system can be calculated by adding and subtracting whole numbers. If a cable loses 6 dB and an amplifier adds 15 dB, the net gain is 9 dB. No multiplication required.
What Your Phone’s Signal Bars Actually Mean
The bars on your phone are a rough translation of the underlying dBm reading, and they’re far less reliable than most people assume. Different phone manufacturers use different scales to map dBm values to bars, and each cellular carrier interprets them differently too. A phone showing full bars on one network might display fewer bars than a different phone model on the same network with identical signal strength. Two bars on an iPhone and two bars on a Samsung phone are not the same measurement.
To see the actual dBm value on an iPhone, open the Phone app and dial *3001#12345#*, then tap the green call button. This opens Field Test Mode, which displays raw signal data. On Android, free apps like SignalStream provide the same type of reading without needing a special code.
Cellular Signal: Four Metrics That Matter
Modern 4G and 5G networks use four distinct indicators to describe your connection, and each one tells you something different.
RSSI (Received Signal Strength Indicator) shows the total signal power your device is picking up, but it includes noise and interference along with the useful signal. Think of it as measuring the total volume in a room without distinguishing speech from background chatter.
RSRP (Reference Signal Received Power) isolates just the useful signal coming from the cell tower. This is the most direct measure of how strong your connection actually is. Industry benchmarks for RSRP break down like this:
- Excellent: -80 dBm or higher, with maximum data speeds
- Good: -80 to -90 dBm, strong and reliable
- Fair to poor: -90 to -100 dBm, with possible drop-outs as you approach -100
- Poor: below -100 dBm, where performance drops drastically
RSRQ (Reference Signal Received Quality) tells you how clean the signal is. A reading of -10 dB or better is excellent, while anything worse than -20 dB indicates a heavily congested or noisy connection. You can have decent signal strength (RSRP) but still experience slow speeds if the signal quality (RSRQ) is poor.
SINR (Signal-to-Interference-plus-Noise Ratio) compares the strength of your signal to the interference around it. This is often the best predictor of actual data speed. A SINR of 20 dB or higher means excellent performance. Between 0 and 13 dB, you’ll get usable but inconsistent speeds. Below 0 dB, interference is overpowering the signal, and you’ll experience frequent drop-outs.
Wi-Fi Signal Strength Ranges
Wi-Fi signal strength is reported as RSSI, measured in dBm. The scale works the same way as cellular: closer to 0 is stronger, closer to -100 is weaker. For bandwidth-heavy tasks like video calls or streaming, you generally need an RSSI better than -70 dBm. Below that threshold, performance degrades noticeably.
In practical terms, standing in the same room as your router typically gives you something between -30 and -50 dBm. One or two rooms away, you’re likely in the -50 to -67 dBm range. Once you’re on a different floor or behind thick walls, readings often fall below -70 dBm, where buffering and dropped connections become common.
Why Strong Signal Can Still Mean Slow Speeds
Signal strength and signal quality are two different things, and confusing them is one of the most common frustrations people have with wireless performance. A strong signal that’s full of interference will perform worse than a moderate signal in a clean environment. This is what the Signal-to-Noise Ratio (SNR) captures: the gap between the useful signal and the background noise.
Imagine trying to have a conversation at a concert. The person next to you might be shouting (strong signal), but you still can’t understand them because the music is louder (high noise). The same thing happens with wireless signals. Your phone might show good RSRP, but if the SINR is low, your actual throughput suffers because the device has to constantly re-request data that was corrupted by interference.
Bluetooth and Short-Range Measurement
Bluetooth devices also use RSSI to report signal strength, and the dBm scale works the same way. One common application is using Bluetooth RSSI to estimate distance, which is how “Find My” features and contact-tracing apps work. But RSSI-based distance estimation in Bluetooth is surprisingly imprecise. The signal bounces off walls, furniture, and even the human body, creating inconsistencies that make raw RSSI a poor distance ruler.
Research from Virginia Tech found that combining Bluetooth RSSI with motion sensor data can achieve a median distance error of about 3 cm for on-body sensors and 4 to 7 cm when tested across different users. Without that sensor fusion, accuracy is significantly worse. This is why Bluetooth-based location features give you a general area rather than an exact pinpoint.
What Walls and Windows Do to Your Signal
Building materials absorb radio energy, and the losses vary dramatically depending on what’s between you and the antenna. Every decibel lost means less signal reaching your device, and because the scale is logarithmic, even modest-sounding losses translate to significant real-world impact.
- Brick and stone: -8 to -28 dB loss
- Concrete and cement (6 inches): -10 to -20 dB loss
- Tinted and low-E glass: -24 to -40 dB loss
Energy-efficient windows with low-E coatings are particularly problematic. They’re designed to reflect heat, but they also reflect radio waves. A 40 dB loss means your signal is reduced to one ten-thousandth of its original power. If you’re starting with a fair signal of -85 dBm outside and losing 30 dB through low-E glass, you’re left with -115 dBm indoors, which is effectively no usable connection.
Professional Measurement Tools
For most people, the field test mode on a phone or a signal-measuring app provides enough information. Professionals working with RF systems use more specialized instruments. A basic signal meter reads power levels at specific frequencies, similar to what your phone does but with greater precision. A swept spectrum analyzer goes further by scanning across a range of frequencies and showing power at each one, which is useful for identifying interference sources. Its limitation is that it only works well with stable, unchanging signals.
A vector signal analyzer captures both the strength and timing characteristics of a signal, allowing it to decode modulated data. The most capable tool is a real-time spectrum analyzer, which can detect brief, transient signal events that other instruments miss entirely. These are used to diagnose intermittent interference problems, the kind where a connection drops for a fraction of a second and then recovers before a traditional analyzer can even register what happened.