The lbmlbf Relationship: Why it Matters. Instead of weighing 170 lbs, I would respond saying that I have a mass of 5, 474 lbm fts 2 (. 2) time to diet I think. Of course this is ridiculous. The confusion comes from an overgeneralization, i. e. 12 inches in a foot, therefore 32. 2 lbm in a lbf in not true.