Knot density is a traditional measure for quality of handmade or knotted pile carpets. It refers to the number of knots, or knot count, per unit of surface area - typically either per square inch (kpsi) or per square centimeter (kpsc), but also per decimeter or meter (kpsd or kpsm). Number of knots per unit area is directly proportional to the quality of carpet. Density may vary from 25 to over 1000 kpsi, or 4 to over 155 kpsi, while ≤80 kpsi is poor quality, 120 to 330 kpsi medium to good, and ≥330 kpsi is very good quality. The inverse, knot ratio, is also used to compare characteristics. Knot density = warp×weft while knot ratio = warp/weft. For comparison: 100,000/square meter = 1,000/square decimeter = 65/square inch = 179/gereh.
For two carpets of the same age, origin, condition and design, the one with the higher number of knots will be the more valuable. Knot density is normally measured in knots per square inch (KPSI) which is simply the number of vertical knots across one inch of carpet multiplied by the number of horizontal knots in the same area. Average knot density varies between region and design. A rug could have a knot density half that of another yet still be more valuable, KPSI is only one measurement of quality and value in Persian carpets.
Knot density is related to and affects or affected by the thickness of the length of the pile and the width of the warp and woof, and also the designs and motifs used and their characteristics and appearance. "In rugs with a high knot density, curvilinear, elaborate motifs are possible. In those with a low knot density (as well as kilims), simpler, rectilinear, motifs tend to prevail." "A carpet design with a high knot density is better adapted to intricate and curvilinear designs, which of necessity must have a shorter pile length to avoid looking blurry. A carpet with a lesser knot density is better adapted to bold, geometric designs and can utilize a long pile for softer, more reflective surface that appeals to the sense of touch."
Hand-tying of knots is a very labour-intensive task. An average weaver can tie almost 10,000 knots per day. More difficult patterns with an above-average knot density can only be woven by a skillful weaver, thus increasing the production costs even more. An average weaver may tie 360 knots per hour (1/10 seconds), while 1200 knots approaches the maximum a skilful weaver can tie per hour.
In the late fifteenth century a "carpet design revolution" occurred, made possible by finer yarns, and before this time it is rare to find carpets with ≥120 kpsi but by the next century carpets with three to four times that density were fairly common. For example, the Pazyryk carpet (ca. 400 BCE) is around 234 kpsi and the Ardabil Carpets (ca. 1550 CE) are 300–350 kpsi. A fragment of a silk Mughal carpet in the Metropolitan Museum of Art has a knot density of 2,516 kpsi, while the highest known density is a silk Hereke prayer rug (ca. 1970 CE) with 4,360 symmetric kpsi.
In Persian, reg (raj, rag, Persian: "row, course") refers to the knots per gereh (Persian: "knot"), which refers to a unit of approximately 2.75 inches. Dihari is a unit of 6,000 knots used to measure production in India.
- "Silk Rug Cleaning - Modern and Oriental Rug Cleaning", MrGreenCarpetCare.com. "The quality of an Oriental rug – and all silk rugs in general, is determined by a most important factor: the rug's knot density." Accessed: December 13 2016.
- Goswami, K.K.; ed. (2009). Advances in Carpet Manufacture, p.239. Woodhead Publishing in Textiles: Number 87 (The Textile Institute). ISBN 9781845695859. "Knot density is an indicator of quality; the greater the number of knots per square inch (kpsi), the better the quality of the carpet."
- Cucker, Felipe (2013). Manifold Mirrors: The Crossing Paths of the Arts and Mathematics, p.89-90. Cambridge University. ISBN 9781107354494. "The knot density...not only provides a measure of the work required to produce a given rug but also sets limits to the possible designs."
- Daniel, Elton L. and ʻAlī Akbar Mahdī (2006). Culture and Customs of Iran, p.138. Greenwood. ISBN 9780313320538. "Another basic factor in determining the quality of a carpet is knot density...—more knots indicate finer work, better quality, and a higher price.
- Stone, Peter F. (2014). Oriental Rugs: An Illustrated Lexicon of Motifs, Materials, and Origins, unpaginated. Tuttle. ISBN 9781462911844. Knot density: "In a knotted pile fabric, the knots per unit area. The product of the vertical or warp-wise knot count per linear unit measure multiplied by the horizontal or weft-wise knot count per linear unit measure." Knot ratio: "In a knotted pile carpet, the vertical or warp-wise knot count per linear unit measure divided by the horizontal or weft-wise knot count per linear unit measure."
- (1990). Textile Museum Journal, Volumes 27-37, p.68. "Knot density itself is less significant than the ratio of horizontal to vertical knot counts."
- Murphy, Brian (2006). The Root of Wild Madder: Chasing the History, Mystery, and Lore of the Persian Carpet, p.47. Simon and Schuster. ISBN 9780743264211. "Knot counts appear in almost every carpet book, catalogue, and auction guide. It's a convenient yardstick to compare the intricacy of the work...But it's a seriously flawed method to assess quality or beauty."
- Tzareva, Elena (1984). Rugs & carpets from Central Asia: the Russian collections, p.12-3. Penguin. ISBN 9780140063691. "The greater the knot density, the thinner the weft and warp yarns and the more weakly are they twisted; the smaller the density, the coarser are the foundation yarns." "In small pieces where a high pile was not required, the knot density was often far greater. As less time was needed to weave smaller items, their makers demonstrated greater care, choosing a more complex design that necessitated a greater knot density."
- Denny, Walter B. (2014). How to Read Islamic Carpets, p.43 & 61. Metropolitan Museum of Art. ISBN 9780300208092.