Knots per square inch

What knot density means

Knot density is the number of knots in a set area of a hand-knotted rug. In the United States it is usually given in knots per square inch, or KPSI. Elsewhere you will see knots per square decimeter or per square meter, and in Iran the raj, which counts knots along a gereh of about 2.75 inches.

More knots in the same space means smaller knots, finer yarn and more hours of work. That is why density has long been used as one measure of quality.

How to count the knots

Turn the rug over and lay a ruler along the back, parallel to the fringe. Count the knots in one inch across the width. Then turn the ruler and count the knots in one inch along the length. Multiply the two numbers. Twelve knots across and fourteen along gives 168 knots per square inch.

Each knot usually shows on the back as two small bumps, one on each warp it wraps. Count the pairs, not the bumps, or the result will be twice too high. Count in two or three places and take the average, because density can vary across a rug.

What the numbers tell you

As a rough guide, rugs under about 80 KPSI are coarse, about 120 to 330 is medium to good, and above 330 is very fine. Some silk rugs reach well over 1,000.

High density suits detailed, curving designs such as floral city rugs. Lower density suits bold geometric designs and a longer, heavier pile, which is typical of many village and tribal rugs.

What the numbers do not tell you

Density is only one measure. A coarse village rug with natural dyes, good wool and a strong design can be worth more than a finer rug with harsh colors or weak wool. Age, condition, origin and design all count.

Compare density only between rugs of the same type. Comparing a Heriz with a silk Qom tells you little, because they were never meant to be woven the same way.

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