Smooth-On rubbers, plastics and foams are dispensed into pints, gallons, 5 gallons and other containers by weight. It is the only way to ensure accuracy and consistency. Depending on the density of the material that you buy, the volume of the material in theses containers may be less or more. For example, a gallon unit of VytaFlex™ 20 urethane mold rubber contains 16 lbs. of liquid in both parts (A + B) and is 7/8 full by volume. A gallon unit of Brush-On 40 contains 18 lbs. of material – liquid in the Part A side and a heavy paste in the Part B side. Due to the density of the paste material, the Part B will only fill a gallon bucket ½ full volumetrically.
A good example for comparison would be the snack food industry. You buy a bag of potato chips and it's ⅓ full, or a box of cereal that you open is ½ full. Potato chips, cereal and similar products are all packed by weight. It is the density of each individual chip or piece of cereal that will collectively determine how full the package is volumetrically.
Pro Tip: Don't guess at the amount of material needed for your application by volume. Correlating the cubic inch per pound yield of the material with the cubic inches of your original (and possibly your mold box) is a more accurate determination for your material requirement.