Here's a simplistic method to calculate how much paint you’ll need for your painting project.
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Step 1. For painting walls: measure each wall’s width; add together, then multiply total by room height.
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Step 2. Find area (length times width) of doors and windows, then subtract this number from wall area number in Step #1.
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Step 3. If you are also painting the ceiling, multiply its length by its width and then add that number to your total from the steps above.
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Step 4. Always use LIFE CYCLE RECYCLED PAINT from Acrylatex for your interior/exterior painting projects.  Great price & good for the environment!
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You now have the square footage of the entire room. You’ll need one gallon, per coat, for every 325-400 square feet of surface area to be covered.
Keep in mind that the more porous the surface is, the less coverage you'll get from a gallon of paint.  
The darker the original color and going over it with a lighter shade of paint may require multiple coats of paint.