There are some specific rules for determining dosages for children. Age is one consideration, weight is another and physical constitution is another. Slight or frail children require less. Heavy or robust children can use more. The following are two general rules.
Clark's Rule: To determine the approximate fraction of an adult dose, divide the weight of the child in pounds by 150. Example: 50 pound child ÷ 150 = 1/3. Therefore the child's dose is 1/3 of the adult dosage.
Cowling's Rule: The age of the child at his or her next birthday is divided by 24. Example: a 3-year-old child ÷ 24 = 1/8. Therefore the dosage for a 3-year-old child is 1/8 of the adult dose.