twist

The amount of twist in a yarn plays an important part in determining its character, in particular its hardness or softness and strength. Variation in twist will have considerable effect on the appearance of a fabric and shows in the dyeing and finishing. The measure of twist hardness is a number called the twist factor (twist multiplier).

Twist factor (measured in any indirect yarn count system such as cotton) = Twist, Turns Per Inch (T.P.I.) divided by the square root of the yarn count.
or
Twist factor (measured in any direct yarn count system, e.g. Tex) = Twist, Turns Per Inch (T.P.I.) multiplied by the square root of the yarn count.

Sometime Twist factors are calculated on the basis of Turns Per Metre (T.P.M.)