how to remember Rebar weight ?

I feel it is beneficial to remember rebar weight. Especially those who involve in Taking Off. I learned this formula from my chinese colleague. Rebar weight = diameter x diameter x 0.006167 Thus ~T10 = 10 * 10 * 0.006167 = 0.616 ~T12 = 12 * 12 * 0.006167 = 0.888 ~T16 = 16 * 16 * 0.006167 = 1.579 ~T20 = 20 * 20 * 0.006167 = 2.466 ~T25 = 25 * 25 * 0.006167 = 3.854 ~T32 = 32 * 32 * 0.006167 = 6.313 ~R6 = 6 * 6 * 0.006167 = 0.222 ~R10 = 10 * 10 * 0.006167 = 0.616 ~R12 = 12 * 12 * 0.006167 = 0.888