How do I calculate fabric for a box?

Question:I work in a fabric store and customers come and ask how much fabrc they need for a box with certain measurement. or they ask how much they need for their mattress.

i know its not volume because they fabric width is different. is there a way to calculate this?

Answers:
You mean to cover the box with fabric?

To determine the fabric yardage, add 2" to both the length and width measurements. Multiply each dimension by two and then add the box's thickness to each.