See how much data WhatsApp uses and reduce it in whatsapp

For a breakdown of the total amount of data you are using on WhatsApp go to Settings -> Data Usage -> Network Usage. 

In the Data Usage menu you can limit the amount of data WhatsApp uses by only allowing the app to auto-download images, audio, video and documents when your phone is connected to Wi-Fi.

You can also select the Low Data Usage mode to reduce the amount of data that's used for WhatsApp calls.