I've commented on people purchasing fabric for curtains, when they ask how much they need. I used to work in a fabric store that had a huge home dec department, so though I don't make draperies for anyone other than myself, I know the basics.
What I have noticed is that people think that if (for example) they have a 36" x 40" window, they need a piece of fabric that is 36x40 plus seam allowances. What they don't realize is this will be too small, you really need a piece at least 1-1/2 to 2x the width of your window, if you just want simple flat panels. If you want fullness in your curtain, you need 2-1/2 to 3x the width, or more!