Carpet durability depends on several factors, the most important are: fiber type, amount of twist on the yarn, pile weight relative to stitch rate and gauge which determines density. Judging durability based on any "one" factor alone can be misleading. A less durable fiber with a good twist and density will out perform a more durable fiber that has poor twist and density. Other factors that contribute more to aesthetics than durability are color, softness, luster of the the yarn, and overall texture of the carpet. Remember, the appropriate pad and good maintenance will always increase the life of any carpet.

Carpets of good durability can be found in all of the construction types. Therefore, a good starting point in carpet selection is simply determining the type or style you like, then you can look at differing durability factors within the style type.

Fibers:
Nylon is an ideal fiber for carpet.  It is strong, has good resiliency, and is available "soft to the touch" fibers. Topical stain treatments are often applied to nylon to increase stain resistance. Nylon is a good choice for all construction types and is available in both staple and continuous filaments.
PET Polyester is generally less expensive than nylon and can be made from reclaimed plastic bottles. PET polyester is considered to be less resilient than nylon but more stain resistant. Is available in both staple or continuous filaments and in "soft to the touch" fibers.
Triexta, no longer called PTT polyester, is reported to have the resilience of nylon, greater stain resistance, is more color fast, and is available in very "soft to the touch" fibers. Another ideal fiber for residential carpets. Triexta is always continuous filament.
Olefin (also called polypropylene) is often used in indoor/outdoor carpets and almost always is a loop-pile type construction.  Olefin is more resistant to stains than nylon, generates lower levels of static, but has less resilience than other fibers.
Wool is a natural fiber and presents a beautiful appearance.  However, wool may not be as resilient at man-made carpet fibers.

Yarn Twist:
Yarn twist, particularly in cut-pile carpet is important to performance.  Generally speaking, the carpets appearance will deteriorate with un-twisting of the yarns. Therefore, yarns with higher twist rates (a tighter twist on the individual yarn) will retain their original appearance longer. Heat setting of the yarn also helps in retaining twist and appearance. Today, almost all yarns used in cut-pile carpets are continuous heatset yarns.

Density:
In carpet, density refers to a numeric representation of how tall the yarns are, how tightly tufted they are, relative to the weight in ounces per square yard. This number may be more meaningful to commercial carpet buyers then residential end users because many popular residential styles are "shaggy" in appearance and are created by using taller yarns, tufted further apart and therefore have lower density ratings. A high density factor does not mean heavy carpet, the highest density carpets are often commercial carpets, that are short, tight, firm and weigh less than most residential carpets. Carpets with higher density ratings use to indicate a firmer or a harder feeling carpet, but with the introduction of soft yarns this is no longer true.

Pile Weight:
The amount of yard used to make the carpet, measured in ounces per square yard. Pile weight certainly is a factor in calculating the density of a carpet, and does have a bearing on durability, but judging by weight alone does not address other major factors for durability such as, yarn twist, fiber type, and texture. Comparing carpet based on weight alone can be misleading. While a heavy carpet may feel thick and rich, if it has poor twist and fiber type, it will not perform will.

Carpet Warranties
The length or duration of the carpet warranty is not a direct indicator of durability or performance. Over the past several years the length of carpet warranties has increased significantly while the construction of the carpet has remained the same. As warrantees increase in length, so do the limitations, requirements, and conditions for maintaining the coverage. While the warranty is often an important factor in the purchasing decision, it is best to focus on construction characteristics for judgments in durability.