The amount of twist in a yarn helps to define the style of yarn- a yarn with a lot of air such as a woolen yarn will have much less twist than a yarn with little air, like a worsted yarn. It also affects the stretchiness of the yarn, strength, the halo of the yarn, and many other attributes. Filling or weft yarns usually have fewer twists per inch because strength is not as important as with warp yarns, and highly twisted yarns are, in general, stronger. Warp yarns have to be stronger so that they can withstand the tension of the loom.
The number of twists per inch can, in plied yarns, be determined by counting the number of bumps in one inch, and divide by the number of singles (the strands plied together to make the yarn). If the picture to the right, for example, was of an inch of 2 ply yarn, then the number of twists per inch would be 6 divided by 2, or three, as there are six bumps, and it is a two ply.
While this method works very well with plied yarns, singles don't have bumps to count. One way to determine the tpi for a single is to add a contrasting color fiber when spinning it, and then count the number of times the contrasting fiber has wrapped around the yarn. Another method is to measure an inch of yarn, and untwist it, counting how many full revolutions it takes until there is no twist left. This can be done by inserting two paper-clips into the yarn, at an inch apart, thus making it easier to count a full revolution. A less precise method is to allow the single to ply against itself: the resulting two ply yarn is about half the number of twists per inch of the single. Thus one can roughly find the number of twists per inch for the single, or one can use the doubled back yarn as a measure.
With yarn that is both thick and thin, than it is best to count over several inches and average them. This is because the number of twists per inch will tend to vary from the thin and thick sections.
where is the Twist Multiplier, also known as or the Twist Factor. This Twist Multiplier is an empirical parametr that has been established by experiments and practice that the maximum strength of a yarn is obtained for a definite value of K. In the case of ring spun cotton yarns, for example, the following values of K have been found to give the best results.
Warp yarns, 35's and less 4.75Warp yarns, 35's to 80's 4.50Warp yarns, 80's to 110's 4.25Filling yarns, medium numbers 3.50