When Apple introduced the iPhone, it also managed to get AT&T and then other carriers to offer unlimited data plans in the United States and in a few other countries. That didn't last, especially as networks became congested with heavy data use.
There are still millions of people grandfathered into old plans that allow unlimited data use, but most of us--and all-new users and network switchers--are either on plans that have a fixed amount of data included in each billing period and then charge fees for overages, or on plans that allow "unlimited" usage, but after a certain amount of data is consumed, the connection is throttled from Mbps to Kbps for the remainder of the billing period.
I'm on a family plan with AT&T that allows 10GB of use per month among all our cellular-enabled devices, and then charges $15 per additional gigabyte. After many months on this plan, we haven't exceeded our allocation.
This excerpt from the ebook, A Practical Guide to Networking and Security in iOS 8, offers a variety of advice on keeping your cellular data usage down. You can have mobile access when you need it without breaking your limits, incurring overages, or paying for more chunks of data--if you ration usage. What you need is a strategy.
You can purchase the full ebook for 25% off its retail price by using coupon code MACW25. The ebook is delivered in PDF, EPUB, and MOBI (Kindle-compatible) formats, and includes free updates for the iOS 8 edition.
Tracking cellular usage on an iPhone
An iPhone shows your locally tracked consumption of cellular data via Settings > Cellular > Cellular Data Usage. This number has two problems:
It's not guaranteed to be accurate. Your carrier's records are definitive. In practice, it's pretty close.
It isn't aligned with your billing period. Rather, it's a total of all data consumed since the last time you tapped Reset Statistics at the very bottom of the Cellular or Cellular Data view.
You can, of course, visit your carrier's Web site and get usage information that's typically accurate to within 24 hours, sometimes much less.
If you'd like this number to be more useful, set yourself a reminder in your calendar for the first of each month (or the start of your billing period if it's another increment) to visit Settings > Cellular and tap Reset Statistics.
Check usage in Settings
You can find out how much data you've used just via Personal Hotspot in the Cellular/Cellular Data view. Tap System Services at the bottom, and all the iOS uses, including Personal Hotspot, are displayed.
Check cellular usage on an iPad
A Wi-Fi + Cellular iPad doesn't track cellular usage, because service is typically only sold in time and bandwidth units. The Settings > Cellular Data > View Account screen shows details from the carrier, including the billing period, how much data is included, and the data consumed so far in that period.
Turn cellular data on only when you need it
There are times when you'd prefer not to have an active cellular connection or cellular data link on an iPhone or cellular iPad, notably when you're close to the maximum of your monthly service plan or traveling outside an area included in your data plan (out of the country or in certain remote areas, typically). You can change how the cellular radio interacts with a network in two ways:
To turn off data only, in Settings > Cellular Data (iPad) or Settings > Cellular (iPhone), set the Cellular Data switch to Off. This disables the data link only. On an iPad, that's the entire link to a mobile broadband network; for an iPhone, you can still place and receive voice calls and send and receive SMS/MMS text messages.
To shut off the entire cellular connection, set Airplane Mode to On in the upper left of the main Settings screen, or tap the Airplane Mode button in the Control Center. Airplane Mode turns off all radios, not just cellular. It also dramatically extends your battery life in most cases.
You can also control other cellular data parameters:
Setting Enable LTE to Off will eliminate use of 4G LTE networks and rely on slower 2G and 3G networks. This is useful when LTE networks near you are spotty and you're having trouble staying connected as your device swaps back and forth between 2G/3G and 4G LTE. This can also reduce battery consumption in some cases.
In some markets, the Enable LTE option may read Voice & Data, and let you pick 2G, 3G, or LTE as network options.
Data Roaming can ensure that you don't consume cell bytes while you're outside the home area for your carrier. In some cases, you might have limits; in others, you might be charged. For instance, Sprint and Verizon allow roaming across their networks in areas they don't serve, but limit use to no more than 300MB per month.
Limit your activities on the cell network
Unless you are connected with Wi-Fi, limit your Internet-related activities to those that don't use much data, such as checking email or viewing Web pages.
Various items in Settings let you limit whether cellular data can be used for an app or activity, including:
Use the options in Cellular Data (iPad) or Cellular (iPhone) to prevent excessive use of certain services from consuming a lot of your data allocation. You can turn on and off specific apps, and see their data consumption.
In the Safari settings, you can disable syncing the reading list, which is relatively low bandwidth depending on how you use it.
In iCloud > iCloud Drive, swipe to the bottom and you can disable syncing all items in the list over cellular.
In iTunes & App Store, you can choose whether or not to use cellular data for automatic downloads (four different options for things you've purchased and updates), iTunes Match, and iTunes Radio.
You can also enable or disable cellular use via settings within certain apps. For instance, the podcast app Overcast has a cellular data switch in its Downloads area to let you grab a specific episode or download any available episode via cellular whenever it's available.
More generally, you should avoid using or disable the cellular use in Settings for:
Audio-streaming apps, such as those used by radio stations and networks. Usage is generally small, but it can add up.
Video-streaming apps like Hulu Plus, YouTube, Netflix, and Vimeo. It's easy to run through a gigabyte or more in an hour, depending on your device and connection.
Photo-browsing apps like Flickr. Depending on the app, even swiping past a photo might download a megabyte or more.
Your cellular iOS device will warn you if you start running out of data or start to near your current plan limit during a billing cycle.
The Maps app used to consume lots of data because Apple loaded image data from Google to power its software, even after Google switched to offering vector data for plain maps. Vector data uses scale-independent points and arcs and straight lines between them to represent maps, using vastly less data.
Apple's own Maps app and the revised Google Maps app both use vector data. In looking at heavy usage of Google Maps for a three-month period, my iPhone shows only 94MB of data consumed over cellular.