One of the best ways to determine interior paint costs is to use Behr's Paint and Stain Calculator. You can enter the size of your room, the number of doors and windows and it will give you a good estimate of how many gallons of each paint that you need to buy. Then you can quickly do the math of the paint costs. Don't forget to factor in the supplies you may need to buy to paint a room (like paint rollers, brushes, painter's tape and drop cloths). It's also important to adjust the amount of interior paint needed based on what paint color you are covering. If you are going over dark colors with lighter then you may want to buy an extra can based on the size of the project. Lastly, if you know that we going to need 4 gallons we always spend a little more money for the 5-gallon bucket, which is just a little more. By the time you buy 4 gallons, you are only a few bucks short of a 5-gallon bucket. You don't want to run through 4 gallons and need a 5th because then you will have to spend more money than you had to. Let's face it, there will be touch-ups to do and extra paint is handy for that.