Measure the length in feet of each wall you plan to cover with the wallpaper border. Add another foot to each measurement to account for extra paper in case of mistakes. This also allows you to adjust the paper so the patterns line up with each other.
Add the measurements for each wall you'll cover. For a 10-by-12 room in which you plan to cover all four walls, purchase a total of 44 feet, or 48 if you add an extra foot for each wall.
Check the wallpaper border you plan to use to see how many yards come on one spool. Typically a spool contains 5 yards of paper. Multiply this number by 3 feet to determine how many feet are on the spool. A 5-yard spool contains 15 feet of paper.
Divide the measurements for your walls by the number of feet on one spool of paper. For example, a 48-foot room divided by a 15-foot spool equals 3.2 spools.
Round your answer up to the next whole number. For 3.2, round up to 4. This is the number of spools you must purchase to cover the walls with your chosen wallpaper border. You will typically have some leftover paper when you're finished.