Square foot pricing?

I'm creating an online store for a company that is going to be selling print services over the internet. They sell products with set sizes and attributes, but they also sell custom size vinyl banners. They price the banners per square foot, with a minimum charge for small sizes.

The customer should be able to enter a width & height in inches, then have those values calculate an even square footage, which would be used to calculate a price for that banner. How would I accomplish this using zen cart? If it must be custom-coded, what language would probably be best suited for the task? PHP or javascript? Can anyone point me in the right direction, or point me to a solution?

Thanks! Thank you zen cart developers for a wonderful ecommerce solution!