Determine the coverage area for each color and estimate the number of gallons you'll need for each. For odd walls with angled ceilings, make your best guess. If you're not comfortable doing this, measure the wall at its highest height and multiply that by its width. Now subtract the lowest height from the highest height, multiply that number by the width, cut that answer in half, and finally subtract that new number from the original height by width. That should give you the wall area.
After you have done the multiplication, subtract about 40 sq feet per window, and about 60 sq feet per door. Divide by 400 (interior paint covers 400 sq feet per gallon). The number that remains, is how many gallons you will need. If doing multiple wall colors, then you should do this process for each room (or sets of rooms) with a particular color. 

Prep the house. Wash the walls, remove wallpaper, patch, spackle, seal stains, dry and sand before you attempt to paint. Now is also the time to apply painters tape for trimming, lay drop cloths, etc. Remove all outlet and light switch face plates, collecting screws in a zip-top bag (good opportunity to wash the face plates all at once as well).[4] You can also buy your paint at this time. Don't wait until the last minute. It can take hours to mix many gallons of all your colors. Remember that traffic triples at your home-supply and hardware stores on weekends. Buy on a weekday if possible. House Painting
The Ceiling. For ceilings, flat is recommended, except semigloss or satin should be used in bathrooms with showers/tubs. Some customers opt to go with a different finish for ceilings in general, to create a specific design look, i.e. high gloss ceilings on dining room tray ceiling to make a statement, or eggshell ceilings throughout to look modern and make a statement.  House Painting
Susan Williams, an interior designer at Siren Betty Design, first used Gentleman's Gray (2062-20), a dark blue by Benjamin Moore when designing rooms for a local bed-and-breakfast. Shortly after, everyone at her company fell heads over heels for the shade. "We loved it so much that we painted our office wall the same color. We think it looks especially great in a gloss finish—the way the light reflects off it is gorgeous and gives any room a lot of character." A blueish gray this deep gives a serene, comforting vibe, which works well in a bedroom. House Painting
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