Follow this formula for calculating the number of rolls you need. Measure the width and height of the walls. Multiply the width by the height. This will give you the square footage. Divide this by 50 square feet, which is the total usable square footage in a bolt. This will give you the number of bolts needed. If you have a pattern that has a large match, count on 45 square feet per bolt. Deduct one bolt for every four openings (for example, a
door, a 50" x 60" window etc.) This will give you the total number of bolts needed.