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
tex x 9 = denier; denier / 9 = tex. Example: 7.78 tex = 70 den (common for stockings). The converter above uses this exact factor.
den = tex × 9
Example: 1 tex = 9 den
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).