Calculating the dry density of a substance allows you to know how much a given amount of the substance will weigh. In general, density measures the compactness of a material in pounds per cubic foot. Dry density is used for materials that are frequently soaked with water to compare wet density. Dry density materials include cement, soil and mulch, among other substances. When the material becomes wet, its density increases because of water content.
Things You'll Need
 Scale
 Cardboard box
 Tape measure
 Calculator

Determine the weight of the empty cardboard box in pounds using a scale. For example, assume the cardboard box has a weight of 2 pounds.

Fill the cardboard box with the dry material. If you don't have enough to fill the box, that's alright; just make sure to level the top of the material inside the box.

Use a scale to find the weight of the filled box. Subtract the weight of the box to obtain the weight of the dry material in pounds. Assuming the filled box weighs 25 pounds, then the weight of the dry materials equals 25 pounds minus 2 pounds, or 23 pounds.

Use a tape measure to determine the length, width and height of the dry material inside the box in inches. For example, you might have a length of 20 inches, a width of 15 inches and a height of 10 inches.

Multiply the length times the width times the height to get the volume of the dry material inside the box in cubic inches. In the example, multiply 20 inches by 15 inches by 10 inches, which results in a volume of 3,000 cubic inches.

Convert the volume to cubic feet by dividing the total by 1,728, since a cubic foot equals 1,728 cubic inches. Completing this step, you divide 3,000 cubic inches by 1,728 cubic inches per cubic foot, for a volume of 1.7 cubic feet.

Divide the weight by the volume to obtain the dry density of the material in pounds per cubic foot. In In the example, divide 23 pounds by 1.7 cubic feet, which yields a dry density of 13.5 pounds per cubic foot.
References
 Georgia State University: Density
 "Physics for Scientists and Engineers with Modern Physics"; Raymond A. Serway, et al.; 2009
 Wolfram MathWorld: Volume
 Engineering ToolBox: Volume Units