Finding a Paint's Density
Most retailers list the density of their paint in pounds per gallon, so you won't need to worry about units when doing your conversions. If you have a paint can, it will provide the volume and weight of its contents, so divide the weight by volume to get the density. If you don't have access to the paint can, you can look up on the Internet the type of paint you are using. Most paint companies have easily accessible product information sheets that you can access via a search engine or from the product information on the company's website. These sheets provide detailed information about the product including the density.
Converting Gallons to Pounds
To determine how much a certain volume of paint weighs, multiply that volume by the density. For example, suppose you want to buy 4 gallons of paint with a density of 11.2 pounds per gallon. To calculate this, you multiply 4 times 11.2.
4 gallons x 11.2 pounds/gallon = 44.8 pounds
So, this paint would weigh 44.8 pounds.
Converting Pounds to Gallons
You can also convert the paint's weight to its volume. To do this, divide the weight by the density. Suppose you need 75 pounds of paint, and you want to know how many gallons you need to purchase. If the paint's density is 9.7 pounds per gallon, to know how many gallons that calculates to, you divide 75 by 9.7:
75 pounds / 9.7 pounds/gallon = 7.73 gallons
So, you need to buy at least 7.73 gallons of this paint.