Medication Dosage Calculator
Estimates a safe medication dose for children by scaling an adult dose proportionally to the child's weight, then applying a safety factor. Use it when a pediatric-specific dose is unavailable and weight-based scaling is clinically appropriate.
About this calculator
This calculator uses Clark's Rule-style weight-based scaling to estimate a child's dose from a known adult dose. The formula is: childDose = (adultDose × childWeight / adultWeight) × safetyFactor. The core idea is that drug exposure scales roughly with body mass, so a 20 kg child should receive roughly 20/70 of an adult dose. The safety factor—typically a value ≤ 1—provides an additional conservative buffer to reduce the risk of overdose in a population that metabolizes drugs differently than adults. Note that this is an estimation method; actual clinical dosing must always be verified by a licensed prescriber or pharmacist, as pediatric pharmacokinetics can differ substantially from adults beyond simple weight scaling.
How to use
Suppose a child weighs 25 kg, the adult dose is 500 mg, the average adult weight is 70 kg, and the safety factor is 0.9. Step 1: Divide child weight by adult weight: 25 / 70 ≈ 0.357. Step 2: Multiply by the adult dose: 0.357 × 500 = 178.6 mg. Step 3: Apply the safety factor: 178.6 × 0.9 ≈ 160.7 mg. The estimated safe pediatric dose is approximately 161 mg. Always confirm this result with a healthcare professional before administering any medication.
Frequently asked questions
How does weight-based pediatric dosing work and why is it more accurate than age-based dosing?
Weight-based dosing scales the amount of drug directly to the child's body mass, which is the primary driver of drug distribution volume and clearance rate. Age-based rules (like Young's Rule) use age as a proxy for weight, but children of the same age can vary significantly in size. Because drug concentration in the body depends on how much drug is given relative to tissue mass, weight-based methods produce more consistent therapeutic blood levels. For most medications, mg/kg dosing is now the clinical standard for pediatrics precisely because it accounts for this variability.
What safety factor should I use when calculating a pediatric medication dose?
The safety factor is a multiplier, typically between 0.8 and 1.0, that provides a conservative cushion below the theoretical scaled dose. A value of 0.9 is common for general estimations, reflecting the fact that children's immature liver and kidney function may reduce their ability to clear drugs as efficiently as adults. For narrow-therapeutic-index drugs—such as anticonvulsants or anticoagulants—a lower safety factor or a completely different dosing protocol may be required. Always consult current formularies or a clinical pharmacist to determine the appropriate safety factor for a specific drug.
When should I not rely on a weight-based dosage calculator for children?
Weight-based scaling is an approximation and should not be used as the sole dosing method for drugs with narrow therapeutic windows, neonates and premature infants, or conditions that alter drug metabolism such as liver or kidney disease. It also does not account for age-related differences in receptor sensitivity or protein binding. For chemotherapy, immunosuppressants, and many other critical medications, body-surface-area dosing or direct pediatric pharmacokinetic data should be used instead. This calculator is a screening tool only—final dosing decisions must always involve a qualified healthcare provider.