thumb|Back side of a [[Qom rug with very high knot density]]
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 or higher, where ≤80 kpsi is poor quality, 120 to 330 kpsi is medium to good, and ≥330 kpsi is very good quality. 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.
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 (one every 10 seconds), while 1200 knots approaches the maximum a skilful weaver can tie per hour.
In Persian, reg (raj, rag, Persian: "row, course") refers to the knots per gereh (Persian: "knot"), which refers to a unit of approximately .