If you've upgraded to iOS 9, you should be aware that it comes with a new feature – enabled by default – that could make your next wireless bill a lot bigger.

Wi-Fi Assist automatically switches your phone from Wi-Fi to a cellular connection when the Wi-Fi signal is poor. That's helpful if you're in the middle of watching a video or some other task on the internet that you don't want interrupted by spotty Wi-Fi service.

"But this also means eating through your 500 MB data cap without knowing it," warns the IT department at McGill University in Montreal, which suggests that users may want to disable Wi-Fi Assist.

The new feature means you may be consuming data from your data plan at times when you think you're on Wi-Fi. If your data plan isn't unlimited, that may result in overage charges – in Canada, that's typically five cents per megabyte or $15 per 300 MB that you go over your data cap. (300 MB is what's required to stream a movie for 45 minutes, according to Bell's data calculator).

Chris Mills, a writer for the technology site Gizmodo, wrote that since he started testing the beta version of iOS 9, his data usage has increased by around a third.

"It's impossible to say if that extra usage is directly related to Wi-Fi Assist, but I have my suspicions," wrote Mills Tuesday.

He noted that during the past three days, his data usage has been 950 MB, and half is from Netflix, which he only used at home "using what I thought was WiFi."

So how do you turn Wi-Fi Assist off?

  • Go into Settings.
  • Choose Cellular.
  • Scroll way down to Wi-Fi Assist.
  • Switch the slider to the "off" position.