Pediatric Dosage Calculator

Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer Last updated: August 20, 2026

A Rule From Before Paediatric Pharmacology Existed

Clark's Rule dates from an era when children's doses were routinely guessed at, and its contribution was to insist that the guess should at least be anchored to body weight rather than to age. It divides the child's weight in pounds by 150 — a stand-in for an average adult — and scales the adult dose down by that fraction. It is arithmetic of admirable clarity, and it is also, by modern standards, a historical artifact. Contemporary paediatric dosing works from drug-specific mg/kg figures established for that particular medication in that particular age group, not from a universal ratio.

The Formula

The 150 is the whole argument in a single number: it asserts that a 150-pound person is the adult the adult dose was written for, and that a child is a linearly scaled-down version of that person. Both halves of the assertion are approximations. If you enter a weight in kilograms, this calculator converts it first, multiplying by 2.20462 before applying the rule.

Clark's Rule Applied

Every figure below was computed with the formula above:

The output carries whatever units the adult dose was entered in — the rule is a pure ratio and has no opinion about milligrams versus millilitres. The 75 lb row is exactly half of 150 lb, which is why it returns exactly half of each adult dose.

Where the Rule Falls Down

Linear weight scaling assumes a child is a small adult, and pharmacologically that is not true. Hepatic enzyme systems mature at their own pace, renal clearance in a neonate is a fraction of an adult's per kilogram, body composition and water distribution differ, and the ratio of surface area to mass is markedly higher in a small child. For some drugs these effects roughly cancel; for others they produce large errors in either direction. This is not a defect of the arithmetic — it is a limit of what any single universal ratio can capture.

The Family of Historical Dosing Rules

Clark's is the only one of the three anchored to weight, which is the main reason it has outlived the others in teaching materials even as all three have been superseded in practice.

Why It Still Appears

• Nursing and pharmacy education — the rule remains a standard exam topic and a clean introduction to dose calculation.
• Understanding allometric scaling — it is the simplest possible version of an idea that runs through all of pharmacokinetics.
• Historical literature — older texts reference it directly, and reading them requires knowing what it does.

How to Use This Calculator

1. Enter the Child's Weight as a number.
2. Select the unit — lb or kg. Choosing kg triggers the 2.20462 conversion before the rule is applied.
3. Enter the Standard Adult Dose in whatever units you are working in.
4. Select Calculate. The output shows the converted weight, the ratio, and the resulting child dose in the same units you entered.

Related Calculations

For the modern approach that Clark's Rule was replaced by, see the Medication Dosage Calculator, which works from a drug-specific mg/kg rate. Where surface area rather than weight is the right denominator, use the Body Surface Area Calculator.