I looked into a number of different approaches for calculating cellular data use. Each of the big American carriers offers an app or web tool for monitoring data consumption. But I recommend against those solutions because in my testing, they were generally time-consuming or poorly designed.
Instead, the quickest way to monitor your data use is by simply using your phone. Each carrier has a hidden code that you can punch into your phone to get an update on data use.
For AT&T subscribers, the method is simple. You place a phone call to *DATA# (*3282#). AT&T will send a text message showing the amount of megabytes you have used out of your monthly allotment. It showed that so far this month I have used 464 megabytes out of 3 gigabytes.
[...] Similarly, Sprint customers can send a text message containing the word "Usage" to the number 1311 and get a text message with a data report. T-Mobile subscribers can place a call to #WEB# (#932#) to receive a quick data summary.
(Score: 1) by Francis on Monday December 28 2015, @08:59PM
Same with T-Mobile. They give that information whenever I look at the notifications menu on Android. It seems accurate enough, but even if I go over, that just means the data isn't 4g LTE until the next billing cycle. I rarely need data on my Cell phone, so I don't much nwed the accuracy.