In addition, Android lets you limit background data usage on a per-app basis. While on the Data usage screen, scroll down and you'll see a list of apps that have used a cellular connection, as well as how much data it's used. Tap on the app's name, then scroll down and check the box labelled Restrict background data to keep that app from burning through your plan behind your back.
Set a data cap on Android
If you have an Android phone, you can set an arbitrary mobile data limit: Once you hit the monthly data limit, your phone will prevent any app from using your data connection--they'll have to rely on Wi-Fi instead.
Open the Settings app, and tap Data usage. Next, check the box labelled Set mobile data limit. Once turned on, you can adjust your limit in the data usage chart below the mobile data limit toggle. Use the drag thumb on the right-hand side of the chart to set your data limit, and to tell your phone when you'd like to receive a warning about your data usage.
Turn off Push/Auto-sync
Another way to keep data usage down is to prevent apps from automatically syncing with a remote server.
On Android, Go to Settings > Wireless and network > Data usage, and tap the settings button to toggle Auto sync data on and off. Another way to limit background data usage is to restrict background data for connected accounts on services such as Dropbox, Facebook, Google, LinkedIn, and Twitter. To do so, go to Settings, scroll down to the Accounts section, and tap the account you want to limit. On this screen you'll be able to choose what to sync (or not to sync) and how often; for example, you can elect to sync your Facebook calendar, contacts, or pictures at different intervals.
On iOS, you can combat background data usage by turning off push mail and push notifications. To turn off push mail, or to ensure that your mail app isn't constantly checking for new messages, go to Settings > Mail, Contacts, Calendars and tap Fetch New Data. Toggle the Push switch to off, and set the Fetch schedule to Manually. This way, apps that do not support push notifications will fetch updates only when you open them. The downside? If you use Find My iPhone, you need to leave Push on; otherwise it won't work.
Limit automatic app updates
Both iOS and Android can automatically download and install app updates for you. Yay. If you're on a tight data cap, though, you'll want to make sure that those updates only get downloaded while you're connected to Wi-Fi.
On iOS, go to Settings, then to iTunes & App Store. Next, scroll down and make sure the Use Cellular Data toggle is in the off position. With this turned off, automatic app updates and downloads, iTunes Match, and iTunes Radio will only work over a Wi-Fi connection. (If you want to turn off automatic app updates all together, you can do that, too.)
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