Valerie, if the stairs all run the same direction (no 90º turns at one of these landings), the figure may be a little different than if they all are parallel. This is because you preferably want the carpeting to all run the same direction, but, it can also change direction with the stairs at a landing, unless it has a defined pattern.
To project the amount needed, first measure the width of the stairway, wall to wall, in inches. If one side of the stair is exposed, and is to be upholstered (carpet wraps the exposed edge) include this distance. Now, measure one individual step down the run(tread & riser), again in inches. This is usally about 18". Then measure each landing, including one riser with each.
Count the total number of steps, and multiply your individual step measurement by the total number of steps. Add the measurements of the 2 landings to this.Divide this number by 144. Round the total up to the next whole number (your total will be between 2 & 3). Now times this number by the widest side to side measurement. Add 2". This is the total length of 12' wide carpeting you will need to complete your steps.
I know this may still be a bit unclear; it's much harder to explain than it really is to do. I hope I've made this understandable, but if not, you can post the 3 dimensions (width of stairs, lentgh of 1 step with riser, and landing with riser), and I can easily give you the yardage you'll need.