Some people assume that the number of bars displayed on their iPhone means the strength of the tower signal. In fact, it’s only a general idea of whether the user has a good chance of making a call, according to ars technica. What’s really going on with that meter reveals a lot about how the cell phone system works.

In fact, the number of bars is based on an algorithm that takes many factors into account. After it combines all those elements, it computes how many bars to display. Because the number of bars displayed is an indicator of chances of success, it can be a dynamic value, based on the load on the cell phone tower.

Moreover, different phones and different carriers can utilize a different algorithm to compute the number of bars, according to Jacqui Cheng in her superbly researched article.

Other factors, such as signal to noise can affect the call quality -- such as atmospheric effects or tall buildings that bounce the signal around. If that S/N estimate is too low, the number of bars is reduced and the system won’t let the user make a call.

In summary, while it’s in the best interest of the carrier to have a system that works well and to make the number of bars an accurate guide, there are variables that sometimes undo the best algorithms for the display. In addition, there are no specific industry standards.

"While manufacturers tend to stay within the same general range for each bar of signal, like women’s dress sizes, they don’t often match up exactly. This means that the value can (and does) vary between manufacturers, phone models, and even different firmware versions on the same model," Ms. Cheng noted. In the final analysis, the number of bars is an attempt to provide a rough estimate of your chances of making a call with acceptable sound, not a measured signal strength indicator.

"Just take the bars with a (very large) grain of salt when the service you get doesn’t seem to match up with what the bars tell you, because the relationship between bars and call quality is much more of an art than a science," she concluded.