To effectively work out a complex surface area, as shown left.

Simply place an imaginary grid of 1” squares over the part, and count the whole squares. Then go around the perimeter and add up all the squares that are more than ½ full. Divide the total by 2 and add this to your total of completely filled squares.

Another method is to cover the part with one layer of paper or tinfoil. Then remove the tinfoil and cut into smaller pieces so it can be arranged on the floor as close to a rectangle shape as possible. Then simply multiply length x width of the rectangle.