A new feature in Apple’s iOS 9 could cause a spike in data use

A feature in Apple’s new iOS 9 operating system, designed to keep your connection stable, may be a potential data sink in disguise.

The setting, called “Wi-Fi Assist,” automatically switches your phone over to the cellular network when you’re in a place with spotty Wi-Fi. The feature is on by default in iOS 9, meaning that your iPhones and iPads will seek out cell networks unless you tell them not to.

The problems start when users think they’re somewhere with a good Wi-Fi connection, but their phones beg to differ — leading to bill shock. Users concerned about that can turn the feature off near the bottom of the “Cellular” section of their Settings app.

The iPhone has a built-in counter that helps you keep track of what you’re using. Apple’s customer service forums are a little split on how accurate it is; some say it hews closely to what they see on their bills, while others say it is a little off.

It’s probably best to look at it as an estimate rather than a definitive tally and to check in with your carrier to see how much the company is billing you.

That said, it can be a handy snapshot. To find it, head to that same Cellular menu in your Settings app. Scroll down and you should see a section called “Cellular Data Usage” that gives you an estimate of how much data you’ve used since you last reset that counter.

If you’ve never used this tool before, you’ve probably never reset it. To do so, head to the very bottom of the Cellular menu and hit “Reset Statistics.” You’ll have to reset the counter manually — maybe when your bill comes each month.