🧒 Pediatric Dosage: Clark's Rule

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 1 independent reference calculation · how PhDino checks its numbers

A historical rule of thumb for scaling an adult medication dose down to a child's weight.

Clark's Rule is one of the oldest weight-based pediatric dosing approaches, dating from an era before modern weight-based (mg/kg) dosing guidelines existed for most medications. It scales a known adult dose down in direct proportion to a child's weight relative to an assumed average adult weight of 150 pounds.

Modern pediatric pharmacology has largely moved toward drug-specific weight-based (mg/kg) dosing guidelines derived from actual pediatric studies, rather than scaling down adult doses by simple proportion — children's bodies don't always process medications in the same proportional way adults do, which is exactly what drug-specific pediatric dosing research is meant to capture that a simple ratio cannot.

Key formula

Child's dose = (child's weight (kg) ÷ 68.0) × adult dose
(the classic imperial statement is weight (lb) ÷ 150; 68.0 kg = 150 lb)

Notes & limitations

• Clark's Rule is a historical approximation, largely superseded in modern pediatric practice by drug-specific weight-based (mg/kg) dosing guidelines that come from actual pediatric clinical studies — where a specific pediatric dosing guideline exists for a medication, it takes precedence over this general proportional rule.
• This rule assumes an adult dose scales down linearly with weight alone, which doesn't hold uniformly across all drugs or all age ranges — very young children in particular can metabolize some drugs quite differently from older children or adults, not just proportionally less.