Tampons sizes are based on their absorbency, or the amount of menstrual flow they will comfortably absorb. The United States Food and Drug Administration regulates the sizes used by tampon manufacturers, called ratings, to ensure that all brands at the same rating have the same absorption capacity. The FDA developed five ratings for tampons: light, regular, super, super plus and ultra.
Light absorbency tampons absorb 6 grams of blood or less, and are best suited for times when menstrual flow is the lightest. Generally this would be at the beginning or end of the cycle. Regular tampons absorb 6-9 grams. This is usually a high enough rating for most of the cycle. Super tampons hold 9-12 grams, for times when the menstrual flow is heavier than usual. Super plus tampons absorb 12-15 grams of blood, used for extra heavy bleeding. Ultra tampons absorb 15-18 grams; this is higher than most women need.
While tampons that absorb more than 18 grams are available, there is no FDA rating for them.
Tampons should never be worn for longer than eight hours at a time, due to the increased risk of toxic shock syndrome. Using the lowest rating possible helps reduce the risk.