The specific gravity of the fluid affects Cv calculations because it changes the density of the fluid, which in turn affects the flow rate. For example, a fluid with a higher specific gravity will have a lower flow rate than water at the same pressure drop. To account for this, Cv can be calculated using a formula that takes into account the specific gravity of the fluid, such as the formula: Cv = Q / √(ΔP / SG), where Q is the flow rate, ΔP is the pressure drop, and SG is the specific gravity of the fluid.