Assuming the piece was solid (or a closed welded sheet metal box) using your 5 " x 5" x 12" example:
Bottom: 5 x 5 = 25 sq in.
Top: 5 x 5 = 25 sq. in.
One side: 5 x 12 = 60 sq. in (times (4) sides = 240 sq. in.)
So the total would be 290 sq. in.
Don't forget if you are plating sheet metal to figure both sides of the metal sheet!
George W.
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