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If you’ve been looking at antique Oriental or Persian carpets, then you have probably heard of the term knot per square inch (or centimeter) or knot density – and perhaps wondering what it is.

Knot density is one traditional way of determining the quality of hand woven rugs. It is the number of knots per square area. For example, knots per square inch, or “kpsi”, is simply the number of knots for every square inch. A carpet’s knots are counted on its backside per linear inch, centimeter or decimeter along the carpet’s warp and weft – which are visible on the backside. Then, these two numbers are multiplied.

oriental carpets, London, UKA rug’s knot density is one way in determining its value. However, it is not always true that a rug with a very high kpsi is valuable or of high quality. While knots play a role in the quality of workshop- and city- made rugs, the same is not always true for nomadic and village rugs. Other factors are considered to determine their quality, such as how the design is created from the weaver’s memory and the dyes and materials used and more. This is because workshops and city weavers have access to more sophisticated tools, and nomadic groups typically migrate very often and do not always have the time to weave high- density knots.

Nomadic and village rugs usually have 25 to 100 kpsi. On the other hand, workshop and city rugs can have up to over 1000 kpsi. Not only do they have better tools, they can also draw cartoons to use as their pattern.

So, does the knot density matter in the rug’s value?

Yes and no! You cannot only use knot density alone as the determining factor of a rug’s quality. But, if you compare two rugs of the same origins and from the same category, and have other similar quality factors, then knot density can help you tell which of those two is more valuable.

Here is something to think about: an authentic vintage Persian and Oriental carpet, every knot is hand made. It takes a highly skilled weaver on average about ten seconds to tie a knot (taking into account the running of the warp), that means six knots per minute or 360 knots per hour. If you do the math, a skillful rug weaver will take 6,480 hours to weave 150 kpsi, 9 x 12 foot rug. If the weavers work 8- hour shifts, that means it will take someone 810 days or 2 and a half years to finish the rug. This size rug is usually made by 2 or three weavers, so the number of hours can be divided into two or three.

That is how much work is put on each and every handwoven rug. Ask about the rug’s knot density the next time you go rug hunting, or visit us at Sharafi & Co and we’d be happy to walk you through the kpsi of the rugs that you like.