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.

### Units

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.