If you have old shoes that you can say that they have a well-chosen size, just look at the length of the insole given in the tab on the tongue, compare the result with the size chart, there you will find your number!
Otherwise, to determine exactly the size necessary to order shoes, measure the foot and your size compare with the dimensions given in the table.
To measure the length of the foot (in a sock!), Put it on the A4 sheet, set the heel at the very end and mark the place where your fingers end.
ATTENTION! Each manufacturer has a characteristic size. This means that, for example, the size of Adidas 41EU does not have to be equal to the size of 41EU Nike. That is why you should look for your size in the tables of the company, whose footwear we choose with a distinction between women's, men's, children's sizes, etc.