Start by adding the length of all the walls to be painted, from one end to the other. Then multiply that number (the perimeter) by the height of the walls, from floor to ceiling. That will give you the square footage of the room to be painted. Measure the total distance (perimeter) around the room.

Total painting area %3D 468 m2.For example, let's say a contractor paints a 12-foot steel roller blind. Now you need to determine what amount of those square feet is surface area that can be painted. Since you use a different paint on the doors and windows, subtract those areas from the total area of the room. Don't worry, just subtract 20 square feet for each door and 15 square feet for each average-sized window in the room.

You end up with a number that is close to the actual area of the wall that you need to cover with paint. In general, you can expect 1 gallon of paint to cover approximately 350 square feet. You need a little more than a gallon if the walls are made of unpainted drywall, which absorbs more paint. You should also consider whether you should paint more than one layer.

If you're painting unfinished, heavily patched or dark-colored walls, plan to apply two coats of paint. Now, the highlight of the math problem. Divide the area of the wall that can be painted by 350 (the square foot coverage on each one-gallon can) to find the number of gallons of paint you need for the walls. You can round off odd numbers if the rest is less than.

Of course, buying in bulk is usually cheaper, so you may find that 3 quarts of paint cost as much as a gallon. The following examples explain the calculations for determining the amount of paint needed for a 14 x 20 foot room, which is 8 feet tall and has two doors and two windows. For this example, you want to buy 1 gallon of roofing paint for a single coat. For this example, you want to buy 1 gallon and 2 quarts of paint for a single coat.

For example, imagine that you have ceiling moldings surrounding a room that is 14 feet wide and 20 feet long. Use the same figure to estimate the door coverage you use in your wall area calculations: 20 square feet %3D on a door. Multiply the number of doors by 20, doubling the answer if you plan to paint both sides. Wall paint estimates allow 15 square feet for each window.

It uses approximately half the area of the window to calculate the cutout and the inner frame of the glass is not important for the calculation. Multiply the width by the height to get the total area of each wall. Those two walls together add up to 220 square feet.