What is a WiFi heatmap?
A WiFi heatmap is a floor plan colored by signal strength: green where the signal is strong, red where it is weak. It turns a list of numbers into a picture of where your network reaches and where it does not.
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What the colors show
Each point on the map stands for the received signal strength at that spot, measured in dBm. A measured heatmap is built from readings taken on site; the colors between readings are estimated from the nearest ones. A predictive heatmap is calculated from a plan instead, with no readings at all (more on that below).
Most heatmaps use a scale like the one below. These are the bands WiFi Heatmapper uses.
| 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. |
Understanding dBm
Wi-Fi signal strength is reported as RSSI in dBm, decibels relative to one milliwatt. The values are negative, and closer to zero is stronger: -45 dBm is a very strong signal, -80 dBm a weak one.
The scale is logarithmic. Every 3 dB is roughly half or double the power, and 10 dB is a factor of ten. So the step from -60 to -70 dBm is far bigger than it looks: a tenth of the power. That is why one extra wall can turn a good room into a poor one.
Windows often shows signal as a percentage instead of dBm. The conversion is approximate and differs between drivers, so where you can, use the dBm value from netsh wlan show interfaces.
What counts as a dead zone
A dead zone is an area where the signal is too weak for normal use. There is no single official threshold, but in practice anything below about -75 dBm behaves like one: pages load slowly, calls drop and devices may disconnect. Common causes:
- Distance, especially at the far end of a long flat or office.
- Thick or dense walls: brick, concrete, stone and walls with metal or foil insulation.
- Floors and ceilings between levels.
- Large metal objects: fridges, filing cabinets, mirrors, radiators.
- A router placed in a cabinet, behind a TV or on the floor.
Signal strength is not speed
A heatmap of signal strength shows coverage. It does not show throughput. A strong signal can still be slow when the channel is crowded by neighbors' networks, when many devices share one access point, or when the internet connection itself is the bottleneck. If the heatmap is green and Wi-Fi is still slow, look at interference and the connection, not at coverage.
Heatmap or coverage map?
The two terms are often used for the same thing. When there is a difference, a coverage map usually shows only whether each area passes a threshold (covered or not covered), while a heatmap shows the full gradient. A heatmap tells you more: an area just above the threshold and one far above it look the same on a coverage map.
Measured or predicted
A measured heatmap shows the network as it is, including every wall and every neighbor. A predicted one shows what a model expects before anything is installed. Both are useful at different stages; see predictive vs measured heatmaps.
Make your own
You need a floor plan image and a laptop on your Wi-Fi. The step-by-step guide How to make a WiFi heatmap covers the whole process.