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Well, a tape measure would be a good thing for you to start out with first. But seriously, some panels need more than others, IMO. It does sort of depend on what you're trying to accomplish, other then deadening. Im assuming since you posted this here, its because of a sub install. If so, the box configuration in the cabin would dictate on how much you should use. Some places will proabably need more than just one layer of it. Id probably suggest brownbread, its cheaper and does no different. STFU-80 is also pretty good I hear, however ive never used it. Maybe a little more info would help you let us help you.
I used two bulk packs for a total of 72 sq.ft. to do 1 layer of dynamat extreme for my C5 coupe...rear hatch area, cockpit area, and rear wheel wells. I had one full sheet, 4 sqft, left over.
I did it in single layer only.
Double Dynamat...I bet that is a significant improvement over a single layer. I put a layer of B-quiet's Vcomp on top of my Dynamat...and it adds a sound barrier to the dampening of the Dynamat. Definite improvement over the single layer Dynamat. Here's a thread showing some pictures...
I used brown bread on my 84 coupe. To do the whole interior takes a shop roll (can't remember how much that was, but I know that's what I bought and I had a small scrap left over).