To convert the weight of a liquid into its volume, you first need to know its density. Density is the measurement of how much an object weighs per volume. For instance, formaldehyde has a density of 6.781 pounds per gallon. This means that each gallon of formaldehyde weighs 6.781 pounds. Once you know a liquid's density, you can convert its weight to its volume, or its volume to its weight.
Converting Pounds to Gallons
Say you want to buy 20 pounds of milk. How many gallons should you purchase? The density of milk is 8.6 pounds/gallon. To convert from pounds to gallons, divide weight by the density. Here, you divide 20 by 8.6:
20 pounds / 8.6 pounds/gallon = 2.326 gallons
You need to buy at least 2.326 gallons of milk.
Converting Gallons to Pounds
You can also use density to convert from volume to weight. In this case, you multiply the volume by the density. For instance, take 6 gallons of milk. To figure out how much this would weigh, multiply 6 gallons by the density, 8.6 pounds/gallon:
6 gallons x 8.6 pounds/gallon = 51.6 pounds
Your 6 gallons of milk weighs 51.6 pounds.
Always use the correct density measurement in your conversions. To convert from pounds to gallons, the density you use must be in pounds/gallon units. If the density of your liquid uses different units, then convert the density into pounds/gallon. You can do this quickly by multiplying by the conversion factor for those units. For example, the density of bromine is 3,120.40 kilograms per cubic meter, or kg/m^3. To convert a density in kg/m^3 units to pounds/gallon units, you multiply the kg/m^3 density by the conversion factor of 0.00835. So bromine has a density of 26.055 pounds per gallon.
Also note that the density of a liquid varies slightly depending on its temperature. For instance, water has a density of 8.345 pounds/gallon at 40 degrees Fahrenheit, and density of 8.250 at 120 degrees Fahrenheit. Check the temperature of your liquid to ensure your calculations are as accurate as possible.