TPI is a term used in the textile

Textile

A textile or cloth is a flexible woven material consisting of a network of natural or artificial fibres often referred to as thread or yarn. Yarn is produced by spinning raw fibres of wool, flax, cotton, or other material to produce long strands...

 industry. It measures how much twist a yarn has, and can be calculated by counting the number of twists in an inch of yarn.

Variation per yarn

Twist is needed in yarn to hold the fibres together, and is added in both the spinning

Spinning (textiles)

Spinning is a major industry. It is part of the textile manufacturing process where three types of fibre are converted into yarn, then fabric, then textiles. The textiles are then fabricated into clothes or other artifacts. There are three industrial processes available to spin yarn, and a...

 and plying

Plying

In the textile arts, plying is a process used to create a strong, balanced yarn. It is done by taking two or more strands of yarn that each have a twist to them and putting them together. The strands are twisted together, in the direction opposite that in which they were spun...

 processes. The amount of twist varies on the fibre, thickness of yarn, preparation of fibre, manner of spinning, and the desired result. Fine wool and silk generally use more twist than coarse wool, short staples more than long, thin more than thick, and short drawn

Short draw (spinning)

Short draw is the spinning technique used to create worsted yarns. It is spun from combed roving, sliver or wool top- anything with the fibers all lined up parallel to the yarn. It is generally spun from long stapled fibers. Short draw spun yarns are smooth, strong, sturdy yarns, and dense. Short...

 more than long drawn

Long draw (spinning)

Long draw is the spinning technique used to create woolen yarns. It is spun from carded rolags. It is generally spun from shorter stapled fibers...

.

The amount of twist in a yarn helps to define the style of yarn- a yarn with a lot of air such as a woollen yarn will have much less twist than a yarn with little air, like a worsted

Worsted

Worsted , is the name of a yarn, the cloth made from this yarn, and a yarn weight category. The name derives from the village of Worstead in the English county of Norfolk...

 yarn. It also affects the stretchiness of the yarn, strength, the halo of the yarn, and many other attributes. Filling or weft yarns

Weft

In weaving, weft or woof is the yarn which is drawn through the warp yarns to create cloth. In North America, it is sometimes referred to as the "fill" or the "filling yarn"....

 usually have fewer twists per inch because strength is not as important as with warp

Warp (weaving)

In weaving cloth, the warp is the set of lengthwise yarns that are held in tension on a frame or loom. The yarn that is inserted over-and-under the warp threads is called the weft, woof, or filler. Each individual warp thread in a fabric is called a warp end or end. Warp means "that which is thrown...

 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

Loom

A loom is a device used to weave cloth. The basic purpose of any loom is to hold the warp threads under tension to facilitate the interweaving of the weft threads...

. Filament fibers, such as silk, or many synthetics, need only to be twisted slightly to create a yarn.

Handspinning

Handspinners use the number of twists per inch often. Because the amount of twist defines a lot about a yarn, the number of twists per inch is an important measure to recreate a yarn. As a spinner spins, they will often stop every few minutes to check to see that the number of twists per inch are the same throughout the yarn, as well as that the number of wraps per inch (the thickness of the yarn) are the same. Measuring the number of twists per inch while spinning singles also helps the spinner create a balanced yarn plying

Plying

In the textile arts, plying is a process used to create a strong, balanced yarn. It is done by taking two or more strands of yarn that each have a twist to them and putting them together. The strands are twisted together, in the direction opposite that in which they were spun...

. Yarns that have relatively fewer twists per inch tend to have a softer hand but are not as strong as yarns with more twists per inch, such as medium twist or hard twist yarns. Yarns that have a very hard twist, enough so that the yarn will double back onto itself when released from tension are called crepe-twist yarns.

Determining how many twists per inch

The number of twists per inch can, in plied

Plying

In the textile arts, plying is a process used to create a strong, balanced yarn. It is done by taking two or more strands of yarn that each have a twist to them and putting them together. The strands are twisted together, in the direction opposite that in which they were spun...

 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 fibre when spinning it, and then count the number of times the contrasting fibre 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.

In industry

In the industry the number of twists per inch is calculated as:



where is the Twist Multiplier, also known as or the Twist Factor. This Twist Multiplier is an empirical parameter 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

Cotton

Cotton is a soft, fluffy staple fiber that grows in a boll, or protective capsule, around the seeds of cotton plants of the genus Gossypium. The fiber is almost pure cellulose. The botanical purpose of cotton fiber is to aid in seed dispersal....

yarns, for example, the following values of K have been found to give the best results.

   Warp yarns, 35's and less      4.75

   Warp yarns, 35's to 80's       4.50

   Warp yarns, 80's to 110's      4.25

   Filling yarns, medium numbers  3.50