Open Cart Square Foot Calculator

Need a modification on OpenCart template to perform a square footage calculation on flooring then pass a result to the quantity field before add to cart. The products are sold in boxes and each box has its total of square feet, based on user square feet entry and product square feet per box, we need to calculate how many boxes needed, and this value need to be passed to the quantity field before it goes to the cart. Example would be on this site/page: [login to view URL] This is a quick turnaround project and the successful bidder needs to be available via phone. This is first of many project with open cart.