What does dBm mean? Signal strength numbers explained

dBm means decibels relative to one milliwatt. It is the standard way to express dBm signal strength on phones and Wi-Fi, and because your phone receives far less than a milliwatt, the readings are negative. A number closer to zero is stronger, so −70 dBm beats −90 dBm.

Why are signal readings negative?

Zero dBm is defined as exactly one milliwatt of power. A milliwatt is already small, but the radio power that reaches your phone from a mast several hundred metres away is millions or billions of times smaller still.

Decibels describe ratios, and anything below one milliwatt becomes a negative number on this scale. That is all the minus sign means. It does not mean "no signal" or "an error". It is simply the normal range for received radio power.

A useful way to picture it: think of dBm like floors below ground in a very deep building. Floor −60 is near the surface and easy to reach. Floor −110 is deep down, and the lift barely works.

How the decibel scale works

The decibel scale is logarithmic. Instead of adding the same amount of power at each step, each step multiplies it. Two rules of thumb cover most of what you need:

3 dB is about double or half. −67 dBm carries roughly twice the power of −70 dBm.
10 dB is ten times. −70 dBm carries about ten times the power of −80 dBm, and a hundred times the power of −90 dBm.

This is why a drop that looks small on paper can feel large in use. Moving from −85 to −105 dBm is a 20 dB fall, which means the signal is about a hundredth as strong. Walls, floors and distance cause losses of this size all the time.

It also explains why the scale is useful. A plain power figure would need a long string of zeros after the decimal point. dBm turns those tiny values into short, readable numbers.

What dBm signal strength means on a phone

On a mobile network, the dBm figure your phone shows depends on the network type.

4G (LTE) phones usually report RSRP, reference signal received power. It measures a known signal the cell broadcasts, so it is a fair way to compare one place with another.
5G phones report a similar figure called SS-RSRP, based on the 5G synchronisation signal.
Older 3G and 2G networks used RSSI or RSCP, which measure power in slightly different ways.

Because the measurements differ, a −95 dBm reading on 4G and −95 dBm on 3G are not exactly the same thing. Compare readings of the same type.

For a guide to which numbers count as good on each network, see what a good signal strength is in dBm.

What dBm means on Wi-Fi

Wi-Fi uses RSSI, received signal strength indicator, also shown in dBm. The scale reads the same way, but Wi-Fi numbers tend to sit higher because the router is usually close by. In the same room as a router you might see −40 dBm. Around −67 dBm is usually the weakest level where video calls and streaming stay smooth, and by −80 dBm most connections struggle to stay up.

You can learn how to take these readings properly in how to measure Wi-Fi strength room by room.

The other numbers you may see next to dBm

Strength is only half the story. Two other values describe signal quality, and they are measured in dB, not dBm, because they are ratios rather than power levels.

RSRQ (reference signal received quality) shows how clean the 4G signal is compared with everything else your phone hears. Values closer to zero, such as −8 dB, are better than values like −16 dB.
SINR (signal to interference plus noise ratio) compares the useful signal with interference and noise. Higher is better, and positive values are what you want.

A strong dBm reading with poor quality numbers is a common reason for full bars but slow internet. The signal is loud, but it is drowned out by interference from neighbouring cells.

Common mistakes when reading dBm

Treating it like a percentage. dBm does not run from 0 to 100. It is a power scale with no simple maximum.
Reading the minus sign backwards. −110 is weaker than −90, even though 110 looks like the bigger number.
Comparing different phones directly. Antenna design varies, so two phones can report readings a few dB apart in the same spot.
Judging from one value. Readings move constantly. Watch for half a minute and note the typical range.

How CellRadar helps

CellRadar shows dBm signal strength for your mobile connection live, with the network type, carrier, band and cell ID beside it, and puts the number into plain words so you do not need to decode the scale yourself. Its Wi-Fi Meter shows the same kind of live reading for your Wi-Fi as a gauge. Readings stay on your phone, and the history view lets you see how the numbers change over time.

Frequently asked questions

Is a higher or lower dBm number better?

Closer to zero is better. Because the values are negative, −60 dBm is stronger than −80 dBm.

What is the difference between dB and dBm?

dBm is an absolute power level measured against one milliwatt. dB is a ratio between two values, which is why quality measures such as SINR and RSRQ use dB.

Can a signal be 0 dBm or positive?

In theory yes, but your phone will never receive that much. Positive dBm values describe transmit power, such as the output of a router or a mast, not the signal arriving at your device.

How many dBm does a wall cost?

It varies a lot with the material. A thin interior wall may cost only a few dB, while concrete, brick or metal-backed insulation can cost far more.