Though capped data plans come in different flavors, AT&T now offers a fairly standard set of choices: monthly plans of 200 megabytes, two gigabytes, and four gigabytes. The carrier notes that four gigabytes of data translates to streaming standard-quality YouTube video for just over six and a half hours. According to a data calculator offered by AT&T, streaming an hour of music a day comes to just under a gigabyte of usage each month. These numbers, however, refer to services used in isolation. A user who sends e-mail and surfs the Web in addition to watching the occasional YouTube video will use up the data more quickly. (Because Wi-Fi usage doesn’t count toward the cap, a user could set up a mobile device to run on Wi-Fi in home or office, only depleting the data allowance when on the road.)

The disappearance of unlimited data plans means many consumers will need to monitor their data habits. But plenty of factors make monitoring data use extremely confusing. Not only do different kinds of content burn data at different rates, but carriers also give different figures. T-Mobile’s data calculator, for example, says that streaming an hour of music a day will use up about 1.5 gigabytes of data per month–quite different from AT&T’s figures.

Streaming media–very popular with Netflix customers–burns data the fastest, says Dan Hays, a partner and consultant at PRTM, a global management consulting firm. “The densest, largest content [such as music and video] will provide the greatest strain on mobile networks for the same reason that they’re attractive to the cloud.”

“The collision is here,” Hays says. “In many ways, there’s a fundamental disconnect between communications-hungry applications and limited mobile broadband resources.” Part of the problem, he says, is that carriers and consumers are the ones who have to worry about the costs of streaming data–application manufacturers don’t suffer financially if they produce software that is more data-hungry. As apps move to the cloud, he says, “there needs to be some sharing of the cost of that access.”