My newest project, a fixer-upper, needs to be repainted. The living room, 15 feet by 23 feet, has 12 foot ceilings. I wondered how much paint to buy.
By searching Google, I found a website that has a paint calculator. The site, MIstupid.com, provides a lot of useful information. If you have time to spare, you can read about vitamins, use a currency converter, and much more. Surf the content section for information on sports, history, geography, health, technology, etc.
Back to my dilemma and how much paint to buy...
According to the chart, I'll need 4.56 gallons of paint for two coats. I would have guessed (hoped) three gallons would be enough, but I would have underestimated. Instead of racing back to the store for another gallon or two, I'll save time AND money, and buy a five gallon bucket. I'd rather have a little extra than not enough. With any leftover paint, I can always continue down the hall.
Written and Published by Pat Starnes, McIntosh and Assoc., LLC
Pat Starnes, a Realtor since 1988, specializes in Mississippi Residential Real Estate and can assist you with buying or selling in the Jackson MS Metro area. 
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Written and Published by Pat Starnes REALTOR
McIntosh and Assoc., LLC434 Katherine DriveFlowood, MS 39232601-278-4513 - cell601-936-2007 - office
I was a professional painter for quite some time before getting into Real Estate directly. It really depends on if you are switching colors, trying to cover something up, and how long it has been since it was last painted. You can literally feel how dry the walls are after you have done it a while. It is really hard to rely on something to know for sure how much paint you need other than experience. For sure if you are using expensive paint or think it will take more than 2 coats, start with primer. Its almost always cheaper doing several coats of primer and the final product does come out nicer.