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
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