There are two possibilities to pick the sizes: measuring an existing shirt - the one that fits the best - or measuring the own body. The first possibility is easier and can be performed alone; for measuring the body one may need the help of another person. Moreover, for the measures of the chest and the waist taken directly on the body, the customer has to add a margin in cm according to the wished comfort.
LEVELHemd doesn't add any centimeters to the sizes. The shirt is produced exactly with the measures you put in the form.