How to Make a Cardboard Box (with Pictures) - wikiHow
· For example, a 12 inch long piece of cardboard will make a 3" x 3" box. The width of the cardboard forms the height, base, and top of the box. For example, if you want to make a 3" x 3" box out of a 12" x 9" piece of cardboard, you'll use 3" of the width to form the base and top, and the remaining 6" will form the height of the box.