Density is how heavy something is for its size (how much it weighs divided by how big it is). So, something that is small and heavy has a high density.
Density is mass divided by volume. It's usually measured in grams per milliliter or pounds per gallon. To work out the density of something, divide mass (how much it weighs), by volume (how much space it takes up).
Example:
A gallon of water weighs 8.33 pounds, therefore the density is 8.33 pounds/gallon.
It is important to note that density varies with temperature (TTB uses densities at 60 degrees Farenheit). The gauging manual provides densities for ethanol:water solutions across a broad temperature range.
