Make it easy:
A typical gallon of paint will cover 300 to 450 square feet, depending on the smoothness of the surface and the thickness of the coat applied. This coverage is called the "spread rate" and is printed on the paint container.
In general, count on coverage of about 400 square feet per gallon when painting interior walls and ceilings.
To determine how much paint is needed for a specific paint job, calculate the square footage of the area to be painted (for a wall, height multiplied by width). Dont forget to subtract the space taken up by windows and doors. Divide the total square footage by 400 to get the number of gallons of paint needed for the job.
If a room will require more than one gallon of paint, it is a good idea to put all the paint that will be needed into a single large container and mix it together. There are often slight variations in the color of paint in different containers and the difference can be quite noticeable when the paint is applied. Blending the paint in one container will eliminate this problem.