To calculate the density of knitting needles need to create a sample. To do this, choose a pattern that you plan to knit and yarn. Tie a sample of 12 * 12 cm.
This sample slightly stretch and pin with pins, observing a rectangular shape. Then cover it with a damp cloth and allow to dry.
Then count the number of loops and rows on a plot of 10 * 10 cm. Compare your data with the instructions for your chosen yarn.
If your number of loops per square 10 * 10 is greater than the set, then you can take knitting needles more thickly. If less, then try to knit more tightly or take knitting needles thinner.
I want to note that the density of knitting on skeins of yarn is based on data on the front surface and is not mandatory. Rather, in this way you can determine your knitting density for the particular model you have chosen and make the correct calculation of the loops so that the finished product fits nicely, fits and looks great.
Depending on the yarn, for example cotton, the finished sample should be washed in water with the maximum temperature allowed for this yarn and dried. Thus, it is possible to take into account changes in the process of shrinkage of the fabric or vice versa if there are properties to stretch.