Accurate measurements are essential for a shade to properly fit the window opening. Rectangular Shades: Rectangular shades include Standard Rectangular. Top-Down. Top- Down/Bottom-Up. Two-On-One Headrail. Easy Glide. Skylight Shades Measuring procedures are the same for each of these types of rectangular shades. Inside Mount or Outside Mount: How you take your measurements will vary depending upon whether the shade fits within the window opening (inside mount or IB) or overlaps the window opening (outside mount or OB) "Rounding" Your Measurements: Dimensions should be accurate to within 1/8". With outside mounts, round your measurements up or down to the nearest 1/8''. With inside mounts, always round down to the nearest 1/8''. Rounding up could result in a shade that is too wide to fit within the window opening. Measuring for Inside Mounts: Measure the width inside the window opening at the top, middle, and bottom. Measure the height inside the window opening at the left, right, and center. Order the narrowest width measurement and the longest height measurement. Remember to round your measurements down to the nearest 1/8''. Be sure the window opening is deep enough for the type of mounting bracket being used. Measuring for Outside Mounts: Measure the desired width and height. To minimize light leakage, it's recommended that the shade overlap the window opening at least 1 1/2" on each side and at the bottom. When measuring the height, take into account the  mounting bracket. Measuring For Verticals For outside mount: it is suggested that for light and privacy 4" be added on each side of window 2" should be added to the height