Knots per sq cm is a traditional measure for quality of hand-knotted carpets.

Being of the same age and similar design, the higher the number of knots, the higher value will the carpet have. With more than 30 knots fitted onto every square centimeter (1 sq cm = approx. 0.155 sq inch), the whole carpet pattern will look much more precise and oblong. No sharp stairs or pixelization will be spottable. Carpet's surface will also feel more solid to the touch and would guarantee better protection against aging.

Speaking of Persian carpet, having below 30 knots per sq cm is generally considered an average to low quality carpet. While over 50 indicates a fine piece of work.

Hand tying of knots is a very labour-intensive task. An average weaver can tie slightly below 10 000 knots per day. More difficult patterns with an above average number of knots per sq cm can only be weaved by a skillful weaver, thus increasing the production costs even more. It is not uncommon for a 3m x 2m carpet to be weaved for almost a year by a single weaver.