What it is
Denier and tex measure thread thickness, not button diameter. 1 tex = 1 g / 1000 m; 9 denier = 1 tex exactly.
How to use
denier / 9 = tex; tex x 9 = denier. Example: 70 den = 7.78 tex (common for stockings). The converter above uses this exact factor.
tex = den × 0.1111111111
Example: 1 den = 0.1111111111 tex
Common uses
Pantyhose and stocking denier, sewing-thread specs, and embroidery-thread labels. The number is mass per length, not a length and not a button size.
Conversion tips
A larger number means thicker thread. Do not treat denier as a length, and do not mix it with ligne (button diameter).