On stock Android, this option isn’t enabled by default, but it’s easy enough to do. Simply navigate to Settings > Data usage, and press the three-dot options button at the top of your screen. Make sure Show Wi-Fi usage is selected.
This process, of course, may be slightly different depending on your manufacturer skins (like HTC Sesne or Samsung’s TouchWiz) or version of Android.
Going forward, you’ll now have a WiFi tab on your data usage screen, next to the mobile data tab. Are you surprised by how much – or how little – WiFi data you are actually using in a month?