I've always taken a serendipity view of screw gauge when doing rough
stuff as in "hey, I'd better use a bigger screw for this." I need to
cobble together some two bys for a frame for a basement storage rack.
I've got a box of number 9 2 1/2 inchers that ought to do the trick.

Question: Is there a formula for determining the minimum gauge for a
screw if you know the load?