Calculate pediatric drug doses using Young's Rule, Clark's Rule, BSA method, and mg/kg dosing. Compare all four methods side-by-side with safety checks.
The Pediatric Dose Calculator computes appropriate medication doses for children using four established methods: Young's Rule (age-based), Clark's Rule (weight-based), the BSA (Body Surface Area) method, and direct mg/kg dosing. Because children are not simply small adults — their drug metabolism, body composition, and organ maturity differ significantly from adults — specialized dosing formulas are essential for safe prescribing.
Each method has different strengths. Young's Rule is a quick bedside estimate using only the child's age, but it assumes average weight for age and can be inaccurate for children who are significantly above or below average weight. Clark's Rule improves on this by using actual body weight relative to a standard 70 kg adult. The BSA method, using the Mosteller formula, is a surface-area-based estimate that often aligns well with drug clearance and metabolic rate. The mg/kg method is the most commonly used in clinical practice because drug references publish specific pediatric doses per kilogram.
This calculator lets you compare all four methods side-by-side for any drug, includes preset calculations for commonly used pediatric medications (amoxicillin, ibuprofen, acetaminophen, cetirizine), and includes safety checks so the calculated dose can be compared with the adult maximum.
Children do not scale linearly from adult doses, which is why a single shortcut can miss important differences in weight, body size, or formulation. This calculator compares age-based, weight-based, and body-surface-area methods so the result can be checked against a second method before it is used.
Young's Rule: Pediatric Dose = [Age / (Age + 12)] × Adult Dose Clark's Rule: Pediatric Dose = (Weight in kg / 70) × Adult Dose BSA Method: Pediatric Dose = (BSA child / 1.73 m²) × Adult Dose Mosteller BSA: BSA (m²) = √(Height cm × Weight kg / 3600) mg/kg Method: Pediatric Dose = Weight (kg) × Drug-specific mg/kg dose
Result: BSA dose: 214.5 mg; Young's: 147.1 mg; Clark's: 128.6 mg; mg/kg: 450 mg
For a 5-year-old weighing 18 kg and 110 cm tall, BSA is 0.742 m². The BSA method gives (0.742/1.73)×500 = 214.5 mg. Young's rule gives (5/17)×500 = 147.1 mg. Clark's rule gives (18/70)×500 = 128.6 mg. The mg/kg method at 25 mg/kg gives 450 mg. The BSA and Clark's methods cluster in the low-200 mg range, while the mg/kg method gives a higher value specific to amoxicillin's published pediatric dosing.
Before the development of drug-specific pediatric dosing guidelines, physicians relied entirely on empirical formulas to scale adult doses for children. Young's Rule was published in 1813, Clark's Rule in the late 1800s, and the BSA method gained prominence in the mid-20th century. While modern pharmacology now provides specific mg/kg or mg/m² doses for most drugs, these historical formulas remain useful when specific pediatric data is unavailable, particularly in resource-limited settings or for newly approved medications.
The argument for BSA over weight-based dosing rests on physiology: drug clearance (primarily renal filtration and hepatic metabolism) often tracks more closely with body surface area than with weight alone. Organ blood flow, glomerular filtration rate, and basal metabolic rate can all vary with body size. This approach is especially useful in very young or very small children, where weight-based dosing can produce larger relative doses than intended.
In everyday clinical practice, the mg/kg method dominates because drug references provide specific pediatric doses per kilogram derived from clinical trials. The comparison approach offered by this calculator is most valuable when: (1) no specific pediatric dosing data exists for a drug, (2) the child is at extremes of weight for age, (3) multiple dosing references give conflicting recommendations, or (4) teaching pharmacology students about the principles behind pediatric dosing. A practical approach is to cross-reference calculated doses against at least two independent drug references.
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This worksheet compares historical pediatric dosing formulas with a direct mg/kg estimate so the different sizing assumptions are visible.
The BSA (Body Surface Area) method is a surface-area-based estimate that can align more closely with drug clearance than age alone. For routine medications, the mg/kg method from drug references is the most common approach in practice.
Young's Rule requires only the child's age and is useful for quick bedside estimates. Clark's Rule uses actual body weight and can be more tailored than age-only scaling. Neither accounts for differences in body composition (obese vs. lean children), so BSA is often used when a surface-area approach is preferred.
The Mosteller formula calculates Body Surface Area: BSA (m²) = √(Height cm × Weight kg / 3600). It is simpler than the older DuBois formula and has been validated in children. A standard adult BSA of 1.73 m² is used as the reference.
In many cases the result is capped at the adult maximum, even if the formula yields a higher number. This can happen with mg/kg dosing for heavier adolescents. Some exceptions exist for specific drugs under specialist guidance.
No. Neonates (under 28 days) and premature infants have profoundly different pharmacokinetics — immature renal and hepatic function, higher body water percentage, and different protein binding. Neonatal dosing requires specialized references and neonatologist guidance.
Each method uses a different assumption about how drug requirements scale with body size. Age assumes average development, weight assumes linear scaling, and BSA assumes surface-area scaling. For most children near average proportions, the results converge. Discrepancies widen for children who are significantly over- or underweight for age.