HRN EN standards are valid standard in the Republic of Croatia, which refer to the design of pressure vessels. In this paper, a control calculation of the pressure vessel is performed in accordance with the available HRN M.E2. standards, which are mostly taken from the current German AD Merkblatt regulations, although they are not currently in use in the Republic of Croatia. The pressure vessel which is a subject of the calculation is a water tank with a volume of 7 m3 , which operates under an operating pressure od 8 bar and an operating temperature of 20°C. After the theoretical introduction on pressure vessels and their calculation and construction, the control calculation of the wall thickness of the tank elements will be performed, after which the minimum effective wall thickness for the entire tank will be adopted, followed by the calculation of the inspection opening DN400, as well as stress and deformation which occur at an operating pressure of 8 bar and an operating temperature of 20°C. After the control calculation preformed with the operating temperature of 20°C, the calculation of the wall thickness of the tank elements at the operating temperature of 220°C will be performed, after which the new minimum effective wall thickness for the whole tank will be adopted and appropriate conclusions will be made.