On 4/6/2012 11:05 AM, Gramp's shop wrote:
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?

What you also need to ask is if there is a way to determine the quality
of the screw you are using. I know of #6 screws that are stronger than
#10's.

All things be in equal, McFeeleys.com has the specifically information
that you are asking and IIRC their catalog has a chart of this also.