How to Monitor iPhone Application Data Usage
The iPhone supports several applications that use mobile data, such as Safari, instant messaging applications and streaming video. Depending on your connection speed and the application's individual settings, the amount of data used can vary. If you have a limited data plan or pay according to your data usage, you may want to monitor how much data your iPhone's applications use.
Tap "Settings" on your iPhone.
Scroll down and tap "General" on the "Settings" page and tap "Usage."
Scroll down to "Cellular Network Data" to view the amount of data your applications have sent and received. Tap "Reset Statistics" if you want to reset these figures.