When converting feet to inches; there are 12 inches in 1 foot. So if you have 9 feet and want to know how many inches there are in 9 feet, you have to use 12, the number of inches per every 1 foot.
There are other variables but if you subtract for windows and doors and then add for waste they will be pretty close to the same. It's always better to have to much than not enough. You can always send it back.
