How to use Wi-Fi for facetime on iphone?

So, i have an iphone 5 and you can use facetime without Wi-Fi. But i want to use Wi-Fi, because facetime uses A LOT of data. And i used it one day while my Wi-Fi was on, and it still used my data. So how do i make it use my Wi-Fi?

In setting turn off your cellular data and connect to Wi-Fi then use FaceTime. It's strange this happens to you