Find a scrap of carpeting left over from the installation. If the installation was a long time ago, you may have to hunt for a while, but there's always a scrap or two left. If not, go to the carpet dealer and find the brand and a remnant.
Place the remnant on a piece of old plywood or a metal surface that can't be scored by a utility knife. Turn the remnant over so you can see its backing. Measure a 12-by-12-inch square and mark it with a pen.
Cut out the measured square using a sharp utility knife and a straight edge.
Place the square foot of carpet on a scale and weigh it in ounces. Multiply the number by three to get the number of ounces per square yard. If the number is more than 16, convert to pounds (16 ounces per pound). The resulting number is the weight of your carpet.