Multiply carpet cost by the number of square yards to obtain the approximate cost. Padding is estimated the same way. Although you will need slightly less padding than carpet, for rough estimates use the same yardage for padding as you used for carpeting. In some instances the price per square yard includes the padding and installation. In other cases those are additional costs.
Stairs often cost slightly more than flat installations.
Be sure to ask the salesperson. The average residential stair has a tread 10 to 12 inches deep, a riser six to eight inches high, and is three to four feet wide. Different carpet is installed on stairs in different ways, so discuss this with the salesperson or installer. Sometimes allowing extra carpet on stairs lets you move the carpet down the stairs as wear occurs. With this in mind estimate maximum stairway needs, using three-fourths of a linear yard of carpet per stair.