Two objects may appear identical in size and shape, yet one weighs considerably more than the other. The simple explanation is that the heavier object is denser. An object's density tells us how much it weighs for a certain size. For example, an item that weighs 3 pounds per square foot will be lighter than an object that weighs 8 pounds per square foot. Density is useful in calculating the weight of substances that are difficult to weight. You can determine its weight simply by multiplying the density by the size, or volume, of the item.
Write down the volume and the density of the item you're measuring. Include the units of measurement, such as liters, square centimeters or square inches for volume, and pounds per square inch, grams per square centimeter or kilograms per liter for density.
Check that the volume units are the same as the divisor of the density units. If you have a volume in "square inches" and a density of "kilograms per liter," you cannot convert directly into weight. You need to have "liters" and "kilograms per liter" to perform the calculations.
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Multiply the volume by the density. Check your work by doing the calculations at least twice, even if you are using a calculator to lessen the probability of error.
Cancel out the volume unit and the unit from the divisor, or bottom, of the density unit. For example, if you are multiplying "square centimeters" by "kilograms per square centimeter," you will cancel out "square centimeters" from both measurements and be left with only "kilograms."
Write the resulting weight with the remaining unit. Remember that without the unit, the answer is incomplete.
TL;DR (Too Long; Didn't Read)
Liters are generally used to describe the volume of liquids while square units are used for solids.
Don't confuse volume measurements with length measurements. Although square inches use inches as a basis, the two are completely different measurements.