Make sure the object you're measuring is flat and rectangular.
The width may not be given for items measured in linear feet. If this is the case, you need to measure the width in feet using a ruler or tape measure. If you are only able to measure the width in inches, take the number of inches, and divide it by 12 to get the number of feet.
Since you know that the length is 64 linear feet, multiply the width in feet by 64. The result is the square footage of the item.