Carpet dealers define the "face weight" of a carpet as the ounces of fiber attached to the backing material per square yard. This number has nothing to do with the density of the carpet, because a long, sparse pile can have the same weight as a short, thickly stitched pile. The true "weight" of a carpet is the weight of a square yard of carpet plus the weight of its backing. If your carpet is already installed, weight is easy to calculate.
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.