Youve picked your color, but how many gallons will you need for your room? Calculate the wall area of the room; youll need one gallon for 450 square feet. To determine the wall area, multiply the height of each wall by its width (dont subtract for doors and windows unless they take up more than half the wall); total the figures for the walls. Add 10 percent for future touch-ups. For two coats, double the number.