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In this work, different ways of calculating heat input in welding processes are presented. The interpretation of heat input per unit length according to the standards: QW-409.1 of ASME IX, EN ISO 1011-1 and PD ISO/TR 18491 is discussed. The concept of calculating heat input per unit volume is presented. For exemplary butt and fillet welded joints as well as for padding weld, the heat input values were calculated in accordance with the above-mentioned standards and based on the concept of heat input per unit volume. A comparative analysis of the calculated values of heat input was performed. The study showed a lack of consistency between the individual standards in the interpretation of the heat input. In addition, the practical application of the heat input per unit volume method in calculating the actual amount of heat introduced into the weld was justified.