Whether or not you buy roman shades that measure the same as your window depends on which way you plan to mount the shades. There are three different ways to mount roman shades, but for each way, you will have to measure your window precisely.
How to Measure
To measure your window for roman shades, record all of your measurements to the nearest 1/8 inch. First, measure from left to right for the width, and then measure top to bottom for height. For the best accuracy in your measurements, use a steel tape measure and not a cloth tape measure.
An inside mount involves mounting your roman shades inside your window frame. This mounting style provides a clean look, but is not a good choice if your window is out of square, because your shades will look crooked. If you choose this mount, your window should also be about 3 1/2 inches deep to accommodate the shades, or part of the shade will protrude from your window opening. The measurements for this type of mount should fit your window measurements exactly, so measure carefully.
When you're measuring for this type of mount, you should measure the width and height of your window casing in three places (reference 1). Record the narrowest width measurement and the longest height measurement (reference 1).
When you're measuring the depth of your window casing, factor in any object that may obstruct the roman shade, such as window cranks, handles, and locks (reference 2). If the sill protrudes from the bottom of the window, adjust your height measurements so that the bottom of the shade will rest on it (reference 2).
Mounting your roman shades outside of your window casing provides a richer, fuller look that can aid in privacy and make your window look taller than it really is. If you plan to mount your roman shades outside of the window casing, you still need to measure your window exactly, but add about 1 1/2 inches on each side of the width measurement. If you're mounting the shades above molding, add about 2 inches to your height measurement to allow for the head rail.
If your window has trim all the way around it, you might consider mounting your roman shades directly on the trim. You need to measure the exact width of the trim, edge to edge, from left to right, and the section of the trim that will have mounting brackets attached should be at least 2 inches tall. It should also be a flat, solid surface, so the brackets can sit flush with the trim. The top section of the trim should also be completely level, so the shades will hang properly.
For height measurements for trim mounts, measure the trim's outside edge from top to bottom.