As you can read in the document with which you are associated, this value is indicated in dBm. Designing a comparison between dBm and bars is a more complicated problem than you might think - remember the whole story that the iPhone lost a signal when it is held “wrong”? It was basically just a poorly designed dBm-to-bars mapping: http://www.anandtech.com/show/3821/iphone-4-redux-analyzing-apples-ios-41-signal-fix
It is reasonable to assume that this value will vary from -10 to -100 dBm (based on a look at the WiFi Analyzer application and Wikipedia article on dBm ), but you should not use linear mapping. Take a look at the graphics in the iPhone article for an idea of how it should look.