Yarn fineness is an indicator used to measure the thickness of yarn. There are two commonly used calculation formulas in textile industry: the fixed length calculation formula and the fixed weight calculation formula.

Fixed length calculation formula：

1. Tex number, refers to the weight in grams of a 1000 meter long yarn at a common moisture regain.

It is a fixed length unit, and the larger og the weight in grams, the thicker of the yarn. It is commonly used to represent woolen yarn.

2. Denier, refers to the weight in grams of 9000 meters of yarn or fiber at a common moisture regain. It is also a fixed length unit, and the larger of the weight in grams, the thicker of the yarn or the fiber. It is commonly used to represent synthetic filament, real silk, etc.

3. Dtex, refers to the weight in grams of a 10000 meter long yarn at a common moisture regain. 

Dtex=g/（L*10000), where g is the weight of the yarn in grams and L is the length of the yarn in meters. It is a fixed length unit, and the heavier of the gram, the thicker of the yarn. It is commonly used to represent fiber yarns or multifilament yarns.