Pediatric Dose Calculator

About the Pediatric Dose Calculator

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.

When This Page Helps

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.

How to Use the Inputs

1. Enter the child's age in years.
2. Enter body weight in kilograms.
3. Enter height in centimeters (needed for BSA calculation).
4. Select the preferred calculation method.
5. Choose a preset drug or enter a custom adult dose.
6. For mg/kg method, enter the drug-specific pediatric dose per kg.
7. Enter the number of doses per day.
8. Review the calculated dose and compare all four methods in the table.

Example Calculation

Tips & Best Practices

• Always use the BSA method for drugs with narrow therapeutic indices (e.g., chemotherapy, anticonvulsants).
• Weigh children in kilograms — never estimate weight. If unable to weigh, use a Broselow tape.
• Use actual body weight for Clark's Rule and mg/kg method; consider ideal body weight for obese patients.
• Many drugs are capped at the adult dose, but some references allow exceptions for specific indications.
• Verify all calculated doses against a current pediatric drug reference (for example, Harriet Lane or BNF for Children).
• Round doses to practical measurable amounts — especially for liquid formulations.

History of Pediatric Dosing Formulas

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.

BSA vs Weight-Based Dosing

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.

Practical Considerations

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.

Frequently Asked Questions

• How do the pediatric dosing methods differ?

  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.

• When should I use Young's Rule vs Clark's Rule?

  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.

• What is the Mosteller formula?

  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.

• Should the pediatric dose ever exceed the adult dose?

  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.

• Are these formulas valid for neonates?

  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.

• Why do different methods give different doses?

  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.