Always use a steel tape measure, preferably 3/4" to 1" wide. 

• Measure width first, then height.   
• Measure each window separately, even if they appear to be the same size.
• Survey the window area for trouble spots.  Are there any obstructions?  If so, your measurements will have to be adjusted to accommodate them.  

• For inside mount shades, measure the window opening at the top, middle, and bottom, and use the narrowest measurement.  Also measure the window opening on the diagonal.  If the two diagonal dimensions are not exactly the same, it may be more appropriate to specify outside mount shades for optimal light control and privacy.                                                                                
• For outside mount shades, overlap the window opening by a minimum of 3" on each side to prevent light gaps.  Add at least 2 1/4" overlap at the top to accommodate the headrail.

• Be meticulous.  Take exact measurements.
• Don't forget to specify which side you want the controls on.  If necessary, request extension brackets, spacer blocks to clear obstructions, or hold-down brackets for door applications. 
• Remember, if your window is an unusual size or shape, you'll need to include an actual-size template with your order.  We highly recommend you let your designer handle it.                    Dave' Window Fashions