Knot density refers to the number of knots per square inch or per square decimeter.  The higher the number of knots, the finer the weave. Increased knot density is not synonymous with increased quality.  It is only one of the factors to be considered.

Knot density is determined by counting the number of knots per horizontal inch (along the weft) and the number of knots per vertical inch (along the warp) and then multiplying the numbers together. In practice, this is sometimes difficult because individual knots can be difficult to isolate from the back of the rug and it is generally not possible to isolate separate knots from the front of the rug.

Usually warps are level and in the same plane within the rug. If the warps of the rug lie on the same plane, each knot (whether Symmetrical or Asymmetrical) will show on the back of the rug as two nodes. This situation is illustrated in the diagrams shown above.

Sometimes the warps are depressed or offset so greatly that, from the back of the rug, alternate warps are hidden. If this occurs, each knot will show on the back of the rug as a  single node. This is diagramed at the right with an example utilizing symmetrical knots, but this can also occur with asymmetrical knots and different degrees of warp depression or offset When the warp is offset 90 degrees, as in the second example, the alternate warps will lie on top of its predecessor.

 Chinese rugs, the number of rugs running horizontally along a linear foot is determined and this is referred to as a 'line".  For example, a "90-line" carpet has 90 knots per linear foot. Chinese rugs have severely depressed or offset warps, so you will only see one node of the knot on the back of the rug.

The knot count in Pakistani rugs is graded as "10/20" or "12/24" with the first figure being the number of knots per horizontal inch and the second number being the number on knots per vertical inch. These rugs ( with the exception of many Bokharas ) also have severely depressed or offset warps, so you will only see one node of the knot on the back of the rug.