How to Figure Out the Angle of a Corner Window

Draw two 10-inch lines on the wall or window, extending both to the left and right of the point where the two surfaces meet. Use a light-erasing marking pencil; it disappears and does not leave a permanent line. Ensure these lines are straight and equidistant to the floor by using either a tape measure or level.

Place a registration mark on one 10-inch edge of a piece of 8-by-10-inch paper at 5 inches from the left-hand corner. Mark this section of edge, which is 5 inches long as “left,” and the remaining section, to the right of the mark, as “right.”

Hold the edge of the left section of the paper against the line drawn on the wall. The paper is at a right angle; only the edge of the paper touches the wall.

Shift the paper to the right until the registration mark is at the point where the left and right surfaces meet.

Manipulate the right section of paper so the bottom side of the paper is lying flat along the wall or window, above the drawn line.

Fold this section of paper toward you, keeping the left section snug to the left line drawn on the wall. Make the fold follow the wall; by doing so, you create a representation of the angle from the left section of the paper to the registration mark and through the right-hand section.

Crease the fold when you are satisfied it follows the angle of the wall.

Place a protractor on a table with the curve oriented away from you.

Place the registration mark on the paper at the 90-degree mark on the protractor. Ensure the left section edge of the paper is aligned along the bottom line of the protractor. The angled, right edge of the paper lies along the protractor, from the registration point to a point on the curve of the protractor that indicates the angle. The numbers printed are the number of degrees of angle.