Here's the simple way to measure the area that you want worked on:
Start by dividing the area to be worked into squares, rectangles, and triangles, (as in the diagram above)
Area A is a square, with 12-foot sides. Area B is a rectangle, with 18x20-foot dimensions. Area C is a triangle, with 12- and 18-foot sides forming a right angle. (Remember: a right angle is 90 degrees!)
Figure the area of each shape.
Area A is 144 square feet, (12 times 12). Area B is 360 square feet, (18 times 20). Area C is 108 square feet, (18 times 12, divided by 2).
Add the square footage of all areas to get 612 square feet, (144 + 360 + 108)
HINT: Add 5 to 10 percent, to account for the odd shaped areas that fall outside of the perfect squares, rectangles, and triangles.
In the example above, you would add about 60 square feet! 672 (612 + 60).