A capital B usually refers to the area of the base in any volume of surface area formula.
In any volume of surface area formula, a capital B typically refers to the Area of the base. The base of a solid shape is the bottom surface upon which the shape rests. The Area of the base is the measure of the two-dimensional space enclosed within the boundaries of the base. It is often represented by the capital letter B in formulas for calculating the volume or surface area of various geometric shapes.
