Estimate a child's medication dose as a proportion of an adult dose, using Clark's Rule (weight-based) or Young's Rule (age-based).
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This calculator applies two long-established pediatric dosing rules that estimate a child's dose as a fraction of a standard adult dose. Clark's Rule scales by weight, dividing the child's weight in pounds by an average adult reference weight of 150 pounds and multiplying that fraction by the adult dose. Young's Rule scales by age instead, using the ratio of the child's age to their age plus 12. Both rules were developed as simple bedside approximations before more precise pediatric-specific dosing guidelines existed for most medications, and modern pediatric prescribing generally relies on drug-specific pediatric dosing charts or weight-based mg/kg rates when they're available, since those account for each medication's specific pharmacology far more accurately than a generic age or weight ratio. This calculator performs the historical rule's arithmetic only and does not determine what adult dose or method is appropriate for any specific medication or child. Parents and caregivers should never use these historical rules to substitute for a pharmacist's or physician's specific dosing instructions.
Long before pharmaceutical companies published detailed pediatric dosing charts for every medication, physicians needed some practical way to scale an adult dose down for a child at the bedside, without access to drug-specific pediatric research. Clark's Rule and Young's Rule are two of the best-known solutions to that historical problem, and understanding how they work reveals both a clever piece of practical reasoning and exactly why they've mostly been superseded today.
Clark's Rule takes the more physiologically sensible approach of the two by scaling on weight rather than age. It divides a child's weight in pounds by 150 pounds — a rough reference figure for average adult weight — and applies that same fraction to the adult dose. The underlying logic is that drug distribution and processing scale reasonably well with body mass, so a child weighing a third of the reference adult weight might reasonably need roughly a third of the adult dose, as a rough approximation.
Young's Rule takes a different approach, scaling by age using the ratio of age to age-plus-twelve. This formula was designed to produce a curve that starts low for very young children and approaches, but never quite reaches, the full adult dose as age increases — a reasonable shape for a simple rule, but one with an obvious weakness: age is a much less direct predictor of body size and drug processing capacity than actual weight, since children of the same age can vary enormously in size.
Both formulas share the same fundamental limitation: they were designed as rough approximations for an era before most medications had dedicated pediatric research and dosing guidelines. Modern pediatric pharmacology has largely moved past generic age or weight ratios wherever possible, instead relying on medication-specific dosing established through actual pediatric clinical trials, typically expressed as a direct milligram-per-kilogram rate calibrated for that particular drug's behavior in children rather than a generic fraction borrowed from adult dosing.
These historical rules remain worth understanding, both for their historical significance in the development of pediatric medicine and because they illustrate a broader principle that still applies today: children are not simply small adults, and any dosing approach — historical or modern — needs a method for accounting for the real physiological differences between pediatric and adult patients, rather than assuming a single dose scales down uniformly for every child regardless of medication.
They're largely historical approximations; modern pediatric dosing almost always relies on drug-specific pediatric guidelines or weight-based mg/kg rates when available, since these are far more accurate than a generic age or weight ratio for any specific medication.
Weight-based Clark's Rule is generally considered more physiologically meaningful than age-based Young's Rule, since children of the same age can vary enormously in size, but neither replaces a medication-specific pediatric dosing guideline when one exists.
No — this tool illustrates how these historical formulas work, but any actual medication dose for a child should come from a pediatrician, pharmacist, or the medication's official pediatric dosing information, never from a generic age or weight ratio alone.