Dosage Calculator
An adult dose of 500 mg with Clark's Rule for a 20 kg child gives about 147 mg. This dosage calculator estimates pediatric medication doses using four common formulas. Clark's Rule uses body weight, Young's Rule uses age for children 1 to 12, Fried's Rule uses age in months for infants under 24 months, and the weight-based method scales directly by kilograms relative to a 70 kg adult. Select a formula, enter the adult dose, and provide the child's weight or age as needed.
Quick answer
Clark's Rule: child dose equals adult dose times the child's weight in pounds divided by 150.
Child dose
146.975 mg
Formula
Clark's Rule
Expression
(500 × 44 lbs) / 150 lbs
What this tells you
- •Clark's Rule: child dose equals adult dose times the child's weight in pounds divided by 150.
- •Young's Rule: child dose equals adult dose times the child's age in years divided by age plus 12.
- •Fried's Rule: child dose equals adult dose times the child's age in months divided by 150.
- •Weight-based: child dose equals adult dose times child's weight in kg divided by 70 kg.
- •These formulas produce estimates only. Actual dosing must be determined by a healthcare professional.
- •Each formula has age or weight limits and gives different results for the same child.
How to Use
- 1Enter the standard adult dose in milligrams.
- 2Select the dosing formula. Clark's and weight-based need the child's weight. Young's needs age in years. Fried's needs age in months.
- 3Enter the child's weight in kilograms or age depending on the formula.
- 4Read the estimated child dose. Note the formula and expression used.
How It Works
Formula
Clark's: (adult dose × weight lbs) / 150Clark's Rule scales the adult dose by body weight. The adult reference is 150 pounds. A child weighing less than 150 pounds receives a proportionally smaller dose. Young's Rule uses age instead: child dose equals adult dose times age divided by age plus 12. Fried's Rule adjusts for infants: child dose equals adult dose times age in months divided by 150. The weight-based method uses the ratio of the child's weight to 70 kg, the standard adult reference weight.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Clark's Rule
Young's Rule
Dosage Formulas Overview
| Rule | Age Range | Input | Formula |
|---|---|---|---|
| Clark's | All ages | Weight (kg) | (adult × child lbs) / 150 |
| Young's | 1-12 years | Age (years) | (adult × age) / (age + 12) |
| Fried's | 0-24 months | Age (months) | (adult × months) / 150 |
| Weight | All ages | Weight (kg) | (adult × kg) / 70 |
Common mistakes
- Using these formulas as medical advice. They are reference estimates and do not replace a doctor's or pharmacist's dosing decision.
- Entering age in years for Fried's Rule, which expects age in months.
- Confusing Clark's Rule (weight in pounds) with the weight-based method (weight in kilograms).
- Applying Young's Rule outside the 1 to 12 year age range.
Limitations
These formulas provide rough estimates for educational reference only. They do not account for the specific medication, its metabolism, therapeutic window, organ function, interactions, or individual patient factors. Actual pediatric dosing requires a healthcare professional who considers the specific drug, indication, and patient history. Never use this calculator to determine real medication doses for a child. Consult a doctor, pharmacist, or other qualified healthcare provider for any medication decision.
Embed this calculator on your site
Drop this single line where you want the calculator to appear. It is responsive, mobile-friendly, resizes automatically, and is free to use with attribution.
<script src="https://calctide.com/embed.js" data-tool="dosage-calculator" async></script>Preview the embed at /embed/dosage-calculator/.