Paediatric dose calculator (Clark and Young)

Clark rule gives the child dose as weight in pounds divided by 150, multiplied by the adult dose, while Young rule uses age divided by age plus 12. Both assume a child is a small adult, which is why neither replaces a drug-specific mg/kg dose in practice.

When to use the paediatric dose calculator

These appear in examinations and in older references. In practice, paediatric dosing uses published mg/kg figures for the specific drug, which account for how children actually handle it rather than scaling an adult down.

The paediatric dose formula

Clark rule: child dose = (weight in pounds ÷ 150) × adult dose. Young rule: child dose = [age in years ÷ (age + 12)] × adult dose.

Worked example

Adult dose 500 mg for a 40 lb child. Clark: (40 ÷ 150) × 500 = 133 mg. For a 6-year-old by Young: [6 ÷ (6 + 12)] × 500 = 167 mg.

The mistake that costs marks

Both rules assume a child is a small adult, and children are not. Organ immaturity changes clearance in ways body size does not predict, which is why the two rules disagree with each other and why neither replaces a drug-specific mg/kg dose. Note also that Clark rule uses pounds, not kilograms.

Study aid, not a clinical tool

This calculator exists so you can learn the method and check your working for an examination. It is not medical advice and not clinical decision support, and it must not be used to make a decision about a real patient. Verify any result against your local formulary, the product literature and a qualified pharmacist. Our medical disclaimer sets out the full position.

Calculators you will usually need alongside this

Most questions that reach for one of these reach for another a step later, so these are the three that most often pair with it.

• mg/kg dose calculator — Weight-based dose
• Body surface area calculator (Mosteller and Du Bois) — Body surface area
• Ideal and adjusted body weight calculator — Ideal body weight

Keep going

Calculations are only half of what examinations test. The other half is knowing why a drug does what it does, which is what decides whether a number you calculated is even the right number to be calculating. Start with Best pharmacy calculators. The drug class pages cover the mechanism behind the number, or browse the rest of the free calculators and try the practice questions.