Signal strength tells you how loud a WiFi signal is when it reaches a device. Signal-to-noise ratio (SNR) tells you how much louder it is than the background noise. Both matter, but SNR usually explains more about how well a connection will actually perform.
dBm: the unit behind the numbers
WiFi signal strength is normally expressed in dBm, decibels relative to one milliwatt. Received WiFi signals are tiny fractions of a milliwatt, so the values are negative. The closer to zero, the stronger the signal: −55 dBm is stronger than −70 dBm.
The scale is logarithmic, which makes small numbers mean a lot:
- A change of 3 dB is roughly a doubling or halving of power.
- A change of 10 dB is a factor of ten.
- So −60 dBm carries about ten times the power of −70 dBm, and a hundred times the power of −80 dBm.
That is why a wall that costs "only" 10 dB of signal makes such a difference on the far side.
What RSSI is
RSSI stands for received signal strength indicator. Strictly, it is a relative value each chipset maker defines in its own way, and different devices may report different numbers for the same signal. In everyday use, most operating systems and tools convert it to dBm, and people use "RSSI" and "signal strength in dBm" interchangeably.
The practical point is that devices differ. A laptop with good antennas, a phone held in a hand and a barcode scanner in a metal enclosure will each hear the same access point at different levels. Designs should be based on the weakest device that matters, not the best one.
Noise floor and signal-to-noise ratio
Noise floor
The noise floor is the level of background radio energy on a channel when nothing is transmitting: electronic noise, distant transmitters and interference from non-WiFi equipment. In a quiet environment it is typically somewhere around −90 to −95 dBm on 5 GHz. In busy or industrial environments it can be noticeably higher.
A raised noise floor is often a sign of interference from something other than WiFi, such as wireless video, older cordless devices or electrical equipment. Common sources of WiFi interference covers the usual culprits.
Signal-to-noise ratio
SNR is simply the signal minus the noise, in decibels. If a device hears an access point at −65 dBm and the noise floor is −92 dBm, the SNR is 27 dB.
SNR matters because the faster WiFi data rates pack more information into each transmission, which needs a cleaner signal to decode. As SNR drops, devices fall back to slower rates that tolerate errors better, transmissions take longer, retries increase and everyone sharing that channel slows down. A common rule of thumb in WiFi design is to aim for an SNR of roughly 25 dB where voice or video are used and somewhat lower for general data, but the right target depends on the devices and applications in each area.
Rough guide to signal levels
These are general rules of thumb, not standards. Requirements vary by device, application and vendor guidance.
| Signal (dBm) | What it usually means |
|---|---|
| −30 to −50 | Very strong. Common close to an access point. |
| −50 to −67 | Good. Suitable for voice, video and most applications, provided SNR is also good. |
| −67 to −72 | Adequate for general data and web use. |
| −72 to −80 | Weak. Slow rates, more retries, unreliable for real-time traffic. |
| Below −80 | Poor. Connections drop or barely work. |
Why strong signal is not the whole story
A device can show full bars and still perform badly. Common reasons include:
- Co-channel interference: several access points on the same channel heard at once, all taking turns.
- Too many clients on one access point or channel.
- Asymmetry: the access point is loud enough for the device to hear, but the device's weaker transmitter cannot be heard well in return.
- Non-WiFi interference that raises the noise floor.
- Problems upstream, such as a saturated internet connection.
Turning access point power up is rarely the right fix for these. It can make co-channel interference and asymmetry worse.
Working through two readings
Take two spots in the same building where a survey shows the same signal strength of −65 dBm. In the first, an office area, the noise floor is −92 dBm. The SNR is 27 dB, which comfortably supports voice and video, and devices there should connect at high data rates.
The second spot is beside a production line with variable speed drives and an older wireless camera system. There the noise floor on the same channel measures much higher, for example −80 dBm. The SNR falls to 15 dB. Devices hold their connection and show good bars, but they negotiate slower rates, retry more often and use more airtime for every transmission. Users describe the WiFi as "flaky" rather than weak.
Adding an access point beside the line would raise the signal a little, but it would also add another radio to share the channel. The better first step is to find the noise source with spectrum analysis, then move to a cleaner channel or band, relocate or replace the interfering equipment, or shield the access point's view of it. Only then does it make sense to decide whether more signal is needed.
Measuring signal and noise properly
- Measure with, or allow for, the device that matters most, such as a scanner or phone, not only a survey laptop.
- Measure each band separately. Signal, noise and interference differ between 2.4, 5 and 6 GHz.
- Take readings at the height people actually use devices, not at the top of a ladder.
- Record the noise floor per channel, not just one figure for the whole building.
- Repeat readings when the space is busy, because people, stock and equipment change the results.
Measure both. When you look at survey results, read signal strength and SNR together. If coverage looks good but SNR is low, look for interference rather than adding access points.
How ehZee can help
Our surveys measure signal, noise and SNR across your building with AirMagnet Survey Pro, and compare them with targets based on the devices and applications you actually use. Our WiFi surveys service explains what the numbers mean for your network and what to change. To see how these values appear on a survey, read WiFi heat maps explained.
Common questions
Why does my phone show different bars from my laptop?
Devices have different antennas, receivers and ways of converting signal into bars. Bars are a rough indicator only; dBm and SNR readings are more useful.
Is a higher SNR always better?
Yes, a cleaner signal always helps. In practice you design for an SNR that comfortably supports the applications in that area, rather than chasing the highest possible number.
Can I check signal and noise on my own laptop?
Many operating systems and free tools show signal strength in dBm, and some also show noise. They give a useful quick check at one spot, but they reflect only that device and cannot replace a survey across the whole space.
What SNR is too low?
As a rough guide, once SNR falls into the teens, performance drops noticeably and real-time applications suffer. The exact point depends on devices and data rates.