Linear feet is just the distance from one wall to the opposite wall. To get square footage ft2 measure the distance from one wall to the opposite wall, do this again for the other 2 walls, now...
With great difficulty (that is it is impossible) without further information: 216 square feet represent an area; it has two dimensions (like a width and a depth) linear feet (or just feet) represent...
You don't. There's not enough information. Square feet measures area. Linear feet measures length. It's possible to break an area down to two lengths, such as a square with area 4 could be broken...
You can't actually go from feet to square feet because feet is distance but square feet is area. but to calculate for example the square feet in a rectangle you would do base times height.
BF = nominal thickness" (t) x nominal width" (w) x linear feet'(LF)/ 12" then BFx12"/ (t)x(w)= LFso if you have 200 BF of 2x4200'x12"/2"x4"= 2400/8= 300 LF of lumber