One powerful new feature in the Settings app lets you see how much data your apps are using over your iPhone's cellular connection. You can even block specific apps from accessing data over your cellular connection if you prefer, limiting them to Wi-Fi-based data access only.
To find the options, launch Settings, and tap on Cellular.
Scroll down past all the Personal Hotspot, Call Time, Cellular Data Usage, and other options, and you'll get to a Use Cellular Data For section.
The list is sorted alphabetically, unfortunately, and not by cellular data consumption. But you can see how much data each app has used over your iPhone's cellular connection, and use the green slider to disable certain apps from using cellular data at all.
You can tap on System Services to see how much cellular data is used by system components that you can't prevent from using the cellular connection, like DNS services, Time & Location, Siri, mapping, networking, and such.
You can reset the statistics with a button at the very bottom of the screen.
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