Basic IPv4/IPv6 traffic statistics

Is there a way how to see some basic traffic statistics, especially how much data was transferred over a given interface using IPv4 or IPv6 protocols? I am curious how much of my traffic is actually IPv6 compared to IPv4 but it looks a bit nontrivial to find that out.

This is pretty much the same question on the OpenWrt forum, but unfortunately none of the answers seem to do what I am after.