formula to determine cubic feet The formula to determine cubic feet is straightforward. First, measure the length, width, and height of your package in inches. Multiply these three numbers together to calculate the total cubic inches. Then, divide the cubic inches by 1,728 (since there are 1,728 cubic inches in one cubic foot) to convert to cubic feet. For example, if a box is 12 inches long, 10 inches wide, and 8 inches high, the total cubic inches would be 960. Dividing 960 by 1,728 gives you 0.56 cubic feet.