You can find various data calculators on the web, but here are some estimates of how much data various activities would use:
- 1 email (no attachment) 20KB.
- 1 social media post with photo 350KB.
- 1 email (with standard attachment) 300KB.
- 1 minute of streaming video (standard) 2MB (120MB/hr).
- 1 minute of streaming music 500KB (30MB/hr).
- 1 minute of streaming video (HD) 5.1MB (306MB/hr).
- 1 hour surfing the web 15MB.
- 1 app/game/song downloaded 4MB.
There's also an interesting article here, comparing the data usage of free voice call apps: