NOTE: New belts will be likely shorter than the belt you took off your vacuum. Belts lose elasticity from being stretched to almost twice their original length. After a few months the belt will be an inch or so longer than when it was installed. 
Be sure your brush roll will spin freely before installing your new belt. Also make sure the belt is not pinched or restricted before you turn the vacuum on by turning the brush roll by hand before you plug the vacuum in. If your belt is restricted from spinning freely the motor shaft will melt through your new belt. This leads many people to think they received a defective belt, it usually means the belt was installed improperly or the brush roll is not spinning freely as it should. Please ask if you have any questions. 

Replacing belts and cleaning hair and debris from your brush roll at least once or twice per year will greatly improve the cleaning performance of your vacuum.