Why the bars are not enough
The Wi-Fi icon has three or four bars for a range of about 50 dB. A full icon can mean -45 dBm or -65 dBm, and that difference decides whether a video call works in that room. To compare spots, or to make a heatmap, you need the signal strength itself: RSSI in dBm.
iPhone and iPad: AirPort Utility
Apple does not let ordinary apps read the Wi-Fi signal on iPhone, so the reliable way is Apple's own free AirPort Utility. It has a hidden Wi-Fi scanner that shows RSSI in dBm.
- Install AirPort Utility from the App Store. It is made by Apple and free.
- Open the iPhone's Settings, find AirPort Utility (on recent iOS versions under Apps) and turn on Wi-Fi Scanner.
- Open AirPort Utility (close it first if it was already open). It says it found no AirPort base stations and keeps searching; that is normal, because the app is also for setting up Apple's old routers. Tap Wi-Fi Scan at the top right of that screen, then Scan.
- Find your network by its name. The RSSI line shows the signal, for example -58 dBm. Tap Stop to read it while the list stays still.
With a mesh system or several access points, the same network name appears more than once, once per access point. The strongest entry is usually the one your phone is connected to. The scan lists networks as it hears them, so give it a few seconds at each spot before you read the value.
Android: WiFiAnalyzer
Android lets apps read the signal, so a free app does the job. WiFiAnalyzer by VREM Software is open source, has no ads, and is on Google Play and F-Droid.
- Install WiFiAnalyzer (open-source).
- Allow location access when it asks. Android only shares Wi-Fi details with apps that have location permission, and location must be on.
- The network you are connected to is shown at the top, with its signal in dBm, for example -58 dBm.
If the values update slowly, Android's Wi-Fi scan throttling is limiting how often apps may scan. On Android 10 and later you can turn it off in Settings > Developer options > Wi-Fi scan throttling. Other free apps that show dBm include Ubiquiti's WiFiman. Some phones also show the signal in the Wi-Fi list when Wi-Fi verbose logging is turned on in Developer options, but that differs between phone makers.
What the numbers mean
The values are negative, and closer to zero is stronger. These are the bands WiFi Heatmapper uses for its colors (more in WiFi signal strength in dBm):
| Signal | Rating | What it means in practice |
|---|---|---|
| -50 dBm or better | Excellent | Fast and stable everywhere it reaches. Typical within a few meters of the access point. |
| -60 to -50 dBm | Good | Reliable for everything, including calls and HD streaming. |
| -67 to -60 dBm | Fair | Fine for browsing and streaming. -67 dBm is the common minimum for voice calls. |
| -75 to -67 dBm | Weak | Usable, but calls may stutter and speeds drop. |
| -85 to -75 dBm | Poor | Slow and drops out. Treat these spots as dead zones for practical use. |
| below -85 dBm | Unusable | Little or no usable connection. |
Make a heatmap with your phone
WiFi Heatmapper works in the phone's browser, with no app to install and no account.
- Open it on your phone, add a photo of your floor plan, or sketch the walls if you have no plan.
- Stand somewhere and tap that spot on the plan.
- Switch to AirPort Utility or WiFiAnalyzer, read the dBm value, switch back and type it in.
- Repeat in every room. The heatmap fills in as you go.
Switching between two apps at every spot works, but a second device makes it quicker: read the signal on the phone and place the points on a tablet or laptop.
Tips for consistent readings
- Use the same phone for the whole survey. Phones and laptops have different antennas, so their readings can differ by several dB.
- Hold the phone the same way at every spot, at about chest height. A phone in a pocket or covered by your hand reads lower.
- Stand still for a few seconds before reading. The signal moves a few dB from second to second.
- Measure the band you actually use. If 2.4 GHz and 5 GHz share one network name, the phone can switch between them and the readings jump.
On a Mac or Windows laptop, WiFi Heatmapper Pro adds a helper app that reads the signal automatically as you walk; phone browsers cannot do that. For the full process, see How to make a WiFi heatmap.
