The Justice Department estimated [4] that 3.6 percent of state inmates in 2013 had drug possession as their most serious offense. That includes possession charges for all drugs, not just marijuana. To gauge the marijuana-only percentage, we have to go back to data that’s about a decade old.
Links
[1] http://www.politifact.com/truth-o-meter/statements/2015/oct/14/bernie-s/bernie-sanders-says-people-are-getting-prison-sent/
[2] http://www.rethinkpot.org/tags/political
[3] http://www.rethinkpot.org/tags/prisons
[4] http://www.bjs.gov/content/pub/pdf/p14.pdf
[5] http://www.bjs.gov/content/pub/pdf/sisfcf04_q.pdf