Conventional wisdom says that it takes approximately one gallon of paint to cover a room up to 400 square feet, but this is a single-coat estimate is for a small room like a bathroom, according to Glidden.com. But other variables impact how much paint you should buy, such as the size and number of doors to the room, the height of the ceiling, the size and number of windows and whether you have crown molding, wainscotting and other decorative elements.
Your favorite brand of paint most likely has a rough online calculation tool, such as BenjaminMoore.com, as well as a more detailed estimate. To make sure you have enough, round up to the nearest gallon. That way, you’ll have some leftover paint for touch-ups. If there’s any doubt, confirm the amount by taking your room measurements to your local paint dealer.
There are some circumstances where you’ll need more product. If you’re painting dark walls a lighter color or trying to cover stains or odors, you’ll want to cover the area with a primer before you begin painting. Kilz.com recommends that you use primer to cover everything from permanent marker, graffiti, grease and nicotine stains to stains from water damage, rust or ferrous (iron-based) stains. Severe stains such as those caused by fires, pets, smoke, floods or bodily fluids may require the help of professionals.
Be sure to keep notebook or online record of the color, brand and type of paint you choose.