How Many Square Inches In A 2 Litre Bottle?

Divide the weight of the bottle by the weight of one square inch of the bottle. That will give you the number of square inches.

Alternatively, you could use a tailor's tape measure to measure the dimensions of the area of the bottle you are interested in. Then calculate the result.

The method of measurement you choose would need to be appropriate to the reason why you want to know. If you're making things out of the bottle, you may want to use the "alternative" method. If you're just curious about the surface area, the first method suggested might give you a more accurate result.