Do you get charged by square foot or square yard when having carpet installed?

In Europe we all get charged by square meters when having carpet installed. It depends on the store. Not many years ago everyone in the US priced their carpet, pad, and installation by the square yard. Then someone realized they could advertise much lower prices if they changed everything to square feet. Now most flooring is sold by the square foot, but it seems most still install by the square yard (plus a surcharge for steps and odd shapes).