Here's what we've been told. They started out with a fecal fat test. Anna was started out on regular ole Pancrease way back when she was 3 (that's when she was diagnosed)At that time she took 3 w/meals 1-2 w/snacks depending on the amount of fat and protein in the snack. We were told and it has been repeated throughout the years that fat and protein content was what determined how much she was going to need as she got older. Later on around 8 she was switched to MT-16 (still Pancrease). When Anna changed to the MT-16's she was taking 5-6 of the regular Pancrease and was able to drop back on the number of MT-16's because of the amount of Lipase. It was always a guessing game to adjust depending on if what she was eating was extra greasy like pizza or fried chicken. We were also told that if a meal was extra long to either take half of the enzymes before the meal and half during the meal or to take a couple extra if the meal was longer than an hour. That enzymes won't do much good after an hour. Size didn't really have much to do with how many enzymes my girls take. A couple of years ago we wanted to know if Rachel could move up to MT20's so as not to have to take 6 MT16's. They wanted to do a fecal fat test to determine how much she really needed. Yea, you try getting a teenager to cooperate with that one! She opted to just forget it. At this point my girls are 5 almost 6 inches different in height and about 20 pounds different in weight and take the same number of enyzmes. Here is an example too of how different brands work differently on two different people. When we were stationed in Germany we couldn't get Pancrease MT16 so we tried switching them to Creon. Anna couldn't tolerate it but Rachel could. It just didn't seem to work for her. It was too much of a hassle trying to adjust it so we simply had her prescription filled here in the US and mailed to us. Rachel was able to easily switch between the Creon and Pancrease MT16 when we returned. Mostly because it was easier to have them both on the same enzyme. So they take their prescribed dosage and determine if they need more based on how fatty their meal or snack is or if the meal has been extra long and drawn out.