Knots per square Inch (KPSI)
KPSI (Knots Per Square Inch) is a measurement of the density of knots in a hand-knotted rug and is a general indicator of its quality and durability. Knot count can, however, vary substantially depending on many other factors (such as style, origin, etc.), so this measurement is just one aspect to take into account when assessing a rug.
How do you measure it ?
Turn a hand-knotted rug over and count the individual knots you can see running vertically for 1’’ and then horizontally for 1’’. Multiplying these two figures together will give you the overall KPSI.