Load calculation in HVAC is the process of determining the amount of heating or cooling needed for a given space. It takes into account factors such as the size of the space, the number of windows, the insulation levels, and the climate. The load calculation is used to determine the size of the HVAC system needed to adequately heat or cool the space. It is an important step in the design process of any HVAC system, as it ensures that the system is properly sized to meet the needs of the space.