Box windows are measured for shutters in a similar way to angled bay windows using the same small or large L frame. Instead of meeting at a 45° angle, the frames meet at 90° right angles, butting up to each other.
The box window in picture 1 below is made from wood and as such the handles do not protrude. This allows the frame to be fitted tight to the window frame where 46mm or 64mm louvres are chosen (assuming there are no obstructions such as alarm boxes). As with angled bay measuring, space out 15mm if 89mm louvres are chosen and 30mm if you've chosen 115mm louvres. Picture 2 shows you how the actual frames will fit into the openings prior to fixing.
If you need any assistance when measuring your box window, please call us on UK 08454 890 314 / ROI 01 5065574.