* if you are not painting the ceiling, enter a '0' for length and width. Ceiling length:*. Ceiling width:*. Calculating the amount of paint you'll need for any project is simply a matter of coming up with the square footage to be covered. Paint generally covers about 400 square feet, but to be more precise you can consult the manufacturer's advice on the label. Enter the width of each wall to be painted into the calculator and the height of the walls. Include the ceiling if that is to be painted.