UNDERSTANDING THE NUMBERS

The Heat Map app reports your Wi-Fi signal strength in negative numbers, typically ranging from around -80 to -40. This may seem rather arbitrary, but there is a logic to it. The measurements are in decibel-milliwatts (dBm), where 0dBm means the strength of the received signal is equivalent to one milliwatt of power. The scale is logarithmic, so -10dBm represents 0.1mW of power, -20dBm represents 0.01mW and so forth. The dBm scale itself has no upper or lower limit, but most Wi-Fi gear has a maximum radio transmission power of -30dBm. In practice, any measurement above -50dBm can be considered a good, strong signal, and should get you close to the full performance your hardware is capable of. Weaker signals are still usable, but are more susceptible to transmission errors, leading to slower, uneven data transfer. VoIP vendors typically recommend a minimum signal strength of -67dBm for reliable real-time voice communications; at -80dBm you might see intermittent glitches, and at -90dBm a regular Wi-Fi device probably won’t be able to connect to the network at all. This is thus effectively the bottom of the scale, although scientific radiometric equipment can detect much weaker transmissions. If you’re thinking that a negative logarithmic scale that runs from -90 to -30 is a little counterintuitive, you’re not alone. This is why Windows converts the raw signal strength into a percentage. If you want to see raw dBm figures on a Windows machine you’ll need to use a third-party tool; remember that measurements taken on your laptop might not be directly comparable to those you get from a smartphone, owing to different antenna sizes and designs, but the overall coverage pattern should be very similar.