As a landlord, do you ever just guess how much paint you will need to paint a room in your rental unit? Do you ever end up with not enough paint or way too much? A good rule of thumb is to know that a gallon of paint covers a surface area of around 300 to 400 square feet. Determining The Surface Area Of The Walls Of A Room You'll need a tape measure to determine the surface area of a room. The area of one wall can be determined by measuring the height from the floor to the ceiling