Width Measurement: Measure the outside width of your window casing at the top, middle, and sill as shown. Take its largest and add 1-3 inches(recommended, 1/2-1 1/2" each side) for ultra light control and privacy. If you have any questions, please email [email protected]. Length Measurement: Measure your desired height and record it. We recommend adding at least 4-6 inches to allow window exposure when you roll the shades up. *If you prefer maximum exposure when shades are fully rolled up, please mount them 4" above the top of the outside casing. And add at least 2 inches above the top frame for the headrail. For other types of shades, such as bamboo shades and roller shades, the headrail size changes, and they will have their own outside mount height space requirements. Please read this blog .