Enter the number of knots along the warp and weft, and the length of the warp and weft used for counting to determine the knots per square inch.

Knots Per Square Inch Formula

The following equation is used to calculate the Knots Per Square Inch.

KPSI = (KWarp * KWeft) / (LWarp*LWeft)

  • Where KPSI is the knots per square inch
  • KWarp is the number of knots along the warp
  • Kweft is the number of knots long the weft
  • LWarp is the length of the warp (inches)
  • Lweft is the length of the weft (inches)

What are knots per square inch?

Knots per square inch, or KPI for short, refers to the number of knots (or loops) within a square inch of twine. This measurement is used to determine the strength of yarn and string. A higher KPI means a stronger cord.


Tensile strength is one factor that determines how much weight a piece of twine can bear. The tensile strength of a piece of string is measured in pounds-force (lbf), which is simply the amount of force needed to pull something apart. If the tensile strength were 100 lbs, it would take 100 pounds of pressure to break it.

The higher the tensile strength, the more weight a piece of twine can bear.


The number of KPI can also indicate the size (diameter) of the twine being measured. For example, if a piece of string has 200 KPI and you know its diameter is 3/32″, then you see the line has six strands in it. Some fragments of series may have up to 10 strands in them!


Knots per inch is another way manufacturers express the size and quality of their string and twine products. A higher number here means a thicker diameter, which usually results in greater tensile strength

How to Calculate Knots Per Square Inch?

Example Problem:

The following example outlines the steps and information needed to calculate knots per square inch, first determine how many total square inches there are in the entire rope. Then divide this number by the total number of inches in the string.

To determine the total number of inches in your rope, measure an inch on either side of the center of your string. The two measurements should be very similar, if not identical. If they are different, you do not have an even amount of strands in your rope, and you need to recalculate for this.

Once you have determined the length in inches and feet, you can now calculate knots per square inch by dividing this number by your total square inches.

For example, if you have a 6-foot long piece of 1″ diameter nylon rope and it measures 8″ at each end, there will be 48″ in total length of 24″ per strand. If you count one hundred knots along one strand and then divide this into 200 (48×2), you would get 50