The formula
Clark = adult dose × weight ÷ 70
Where it comes from
Before leaflets carried a dose per kilo, the paediatric dose was estimated by scaling down the adult one. Clark scales by weight against a typical 70-kilo adult, Young by age in years and Fried by months of life. All three are early twentieth-century approximations, and none accounts for the fact that a child's metabolism is not an adult's in miniature: an infant's liver and kidneys work at a different pace.
How to work it out by hand
- Clark: adult dose × weight of the child ÷ 70
- Young: adult dose × age in years ÷ (age + 12)
- Fried: adult dose × age in months ÷ 150
- Compare the three: if they are far apart, none of them is reliable
What is worth knowing
The calculator returns all three deliberately, and the gap between the highest and the lowest as well. That gap is the argument: when three methods claiming to compute the same thing differ by a hundred milligrams out of five hundred, what they are telling you is to go and find the real figure. The real figure is the milligrams per kilo on the leaflet, and that is what the other dosage calculator is for. These rules only help when that figure does not exist, which today is almost never.