Reference the density of the object, most commonly expressed in kilograms per cubic meter or pounds per cubic feet. A pound is technically a unit of weight, but since gravity remains relatively constant on earth, density calculations frequently use pounds. As an example, water has a density of 62.4 lbs. per cubic feet.
Measure or reference the volume of the object. Measure the volume of smaller objects by dropping the object into a beaker filled halfway with water. The difference in the water level before and after dropping the object represents the object's volume. You can also calculate volume by measuring the object's dimensions and using volume calculations, such as width times length times height for rectangular objects. Express this measurement in cubic feet.
Multiply the density by the volume to calculate the mass, or weight if using pounds per cubic feet. In the example, a 10-cubic-foot volume of water would weigh 624 lbs.