To determine how much fabric is needed to make a set of curtains, measure the vertical distance from the curtain rod to the floor, then add 18 inches to calculate the length of fabric needed for each panel. Measure the width of the curtain rod and divide by the width of the fabric to determine the number of panels. Multiply the number of panels by the length required for each panel to arrive at the total length of fabric needed.
The extra 18 inches of fabric purchased for each panel takes into account hemming and any shrinking that occurs during pre-washing of the fabric. Make additional adjustments to the length of fabric for each panel depending on the design aesthetic desired. If you would like the curtains to rise above the curtain rod by 1 inch, for example, add 2 inches to the length of fabric, since it will be doubled when hemmed. If you desire the curtain to be above the floor or to pool on the floor, subtract or add fabric to take this into account.
If you like a more gathered look, the number of panels may increase. With a little gathering, a panel may only cover two-thirds of the width of a flat panel, and in a very gathered look, may only cover one-third.