A closed receptacle has to sustain internal pressure variations due to temperature changes. The internal pressure exerts strain onto the walls and closures of the receptacle and may cause it to fail. For packagings for dangerous goods a prototype internal pressure test is normally required. The test pressure is set as the maximum total gauge pressure to be expected in the receptacle multiplied by a safety factor. The total pressure at a given reference temperature is sometimes difficult to determine, due to the variety of influence factors. In this report an accurate method for calculating the pressure in a closed receptacle is derived.