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. Find the square footage of triangular or sloping walls -- such as those where ceilings are vaulted -- by multiplying the length of the base of the triangle by the height at the highest point and dividing by two.

To make sure you have enough paint do not subtract any square footage for windows and doors unless they significantly reduce the square footage of that wall.