Paint calculator tools provided by paint companies or other organizations can help you estimate how much paint to buy. Paint calculators are useful in planning the right quantity of paint to minimize waste, though be advised different calculators produce different results. Each job is different, so it’s always best to take accurate measurements, check the coverage rate of the paint you are buying, and ask for advice at the paint counter.
Also check out these articles about buying the right amount of paint:
Do It Yourself
Old Farmer’s Almanac
The Paint Shoppes