The "deniers" are the unit of measurement that indicates the weight, in grams, of 9000 meters of the yarn used to make the sock.
So what does it mean when you hear that a stocking or pantyhose has 40 DEN? It means that 9000 meters of yarn in those tights have a mass of ("weigh") 40 grams.
To calculate the "denier" of a wire, take any length of this wire, measure its length (in meters) and mass (in grams) and then apply the formula: mass divided by length multiplied by 9000.
On the contrary, to know how long the thread used to make a single sock is, it is sufficient to divide 9000 by its weight (in grams) and multiply it by the DEN. From here you can easily understand how even to make a "normal" pantyhose it takes several hundred meters.
The "denari" - whose abbreviation is "DEN" - are traditionally used to indicate the degree of compression exerted by an elastic stocking.
Given that a greater weight usually corresponds to a greater thickness of the yarn (or a greater amount of elastic material present), a high degree of denier is associated with greater compression, and the sock is less glazed and therefore more opaque.
On the packaging of stockings and pantyhose, the word DEN preceded by a number therefore (indirectly) expresses the different elasticity, i.e. the different level of compression that the stocking or pantyhose exerts on the leg. 40 DEN stockings or tights will therefore have a lighter compression than 70 DEN or 140 DEN ones.