When measuring a bay window for board-mounted treatments, it's important to consider the depth of the window's projection and any protrusions, such as large window handles or air vents.
After deciding on the type of treatment to install in the bay, you can proceed to take measurements for the board.
To measure an angled bay window for board-mounted window treatments, you need to determine the length, width, and angle of the boards.
To find the length of the board, measure from the back wall, starting at the corner and extending to your desired stopping point, which should be just past the window's crown molding.
Now, to measure the angles, take two index cards, some other heavy paper, or a protractor. Lay the two cards on top of one another and place them in the corner of the window. Spread the cards out so there is a straight edge up against each side of the wall or window crown molding. Tape or staple the cards in place. This will give you your angles.
The measurements for the chosen treatment will be determined by the front measurements of the board.