Yarn density refers to the mass per unit length of fibers, single yarns, mesh wires, ropes, etc. It is an indicator that describes the degree of yarn thickness and can be expressed in two forms: fixed length and fixed weight. Line density is a commonly used indicator to describe the thickness of yarn, and it is measured by the length and mass (g/km). The linear density can be obtained by dividing the fiber or yarn mass by its length.

The linear density of fibers refers to the degree of thickness of the fibers. Linear density is one of the important physical and geometric characteristics of fibers, which not only affects textile processing and product quality, but is also closely related to the wearability of fabrics. Similarly, line density is also the most important indicator of yarn. The legal unit is tex, which is one thousandth, one tenth, and one thousandth of tex, respectively called mtex, dtex, and ktex. The higher the linear density, the thicker the fiber or yarn. The linear density of yarn affects the physical and mechanical properties, feel, style, etc. of textiles, and it is also an important basis for fabric design. The determination of linear density for cotton fibers and synthetic fibers generally uses the mid section weighing method, while the testing of linear density for yarns generally uses the oven drying method (dry weighing).

The linear density of fibers and yarns can be expressed in various forms, generally using indirect indicators proportional to the cross-sectional area of the yarn. The commonly used indicators include tex (number), metric count, imperial count, and denier (the unit of fineness (the degree of fineness of natural or chemical fibers). The weight of a 9000 meter long fiber is determined by its denier. The smaller the denier, the finer the fiber. In general, the linear density indicators of fibers and yarns are mainly divided into two categories: fixed length and fixed weight.

The diameter can also be used to represent the linear density of the yarn. The diameter of yarn is an important basis for fabric design and the formulation of manufacturing process parameters. It can be measured under a microscope, but in actual production, the diameter of yarn is obtained by converting the yarn's characteristics or count and its density.

In the representation of line density of strands, the special number system is represented by multiplying the number of single yarns that make up the strands by the number of splices, such as 142. When the number of individual yarns in the strand is different, it is represented by adding the number of individual yarns together, such as 16+18. The count system is expressed by dividing the count of individual yarns that make up the strands by the number of strands, such as 50/2. If the number of individual yarns that make up the strands is different, the number of individual yarns should be arranged in parallel and cut with a diagonal line, such as 24/48.

The multifilament density of chemical fibers is expressed in terms of the number of single filaments that make up the multifilament and the total tex number. For example, 16.5 tex/30 f indicates that the density of the composite filament is 16.5 tex and the number of single filaments is 30. The multifilament density of chemical fibers or silk is the sum of the single filament densities that make up the multifilament.