Clark's rule is a medical term referring to a mathematical formula used to calculate the proper dosage of medicine for children aged 2–17. The procedure is to take the child's weight in pounds, divide by 150 lb, and multiply the fractional result by the adult dose to find the equivalent child dosage.
For example: If an adult dose of medication calls for 30 mg and the child weighs 30 lb. Divide the weight by 150 (30/150) to get 1/5. Multiply 1/5 times 30 mg to get 6 mg.
Clark's rule is not used clinically, but it is a popular dosage calculation formula for pediatric nursing instructors.
Similar to Clark's rule is Fried’s rule, by which the formula is modified to be used for infants. The formula is nearly identical, except with the child's weight replaced by the infant's age in months.
The formula was named after Dr. Cecil Belfield Clark (1894–1970), a Barbadian Physician who served in London Boroughs for 50 years and was an early advocate for homosexuality rights.