Quality's true measure is performance, a product of the carpet's fibers, construction and density. Face weight, measured in ounces per square yard of carpet, is the amount of face yarn fiber. Generally speaking, the more yarn the better in a given height of pile--a 50-ounce nylon saxony is likely to be better than a 40-ounce nylon saxony if both have a 1/2-inch pile and the same properties. But quantity isn't always quality, therefore face weight isn't really the best measure: density is.
Density addresses the size of the yarn and the number of tufts in each square yard of carpet. The more tufts, the denser and more durable the carpet. A dense carpet is resilient underfoot and resists wear and matting. When shopping, compare samples of carpet by bending them at right angles to see how much backing is revealed.
With a cut-pile carpet, also look closely at the ends of individual yarns. Be sure the fibers are twisted neatly and tightly, and that ends don't flare. Don't be fooled by swatches that fluff-out on the sample rack; tightly-twisted, heat-set yarns will fare far better over time. Also be sure to study and compare warranties of carpets you're considering.
