Pediatric Dose Calculator
Compare Clark's rule, Young's rule and the body surface area method — and see why none of them should replace a published paediatric dose.
Pediatric Dose Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Scaling rules ignore organ maturity, body composition and drug-specific clearance.
- Invalid for neonates and infants, where clearance changes week by week.
- Cannot substitute for a published paediatric dose or a formulary.
In short: For a 6-year-old weighing 21 kg, scaling a 500 mg adult dose gives 150 mg by Clark's rule, 166.7 mg by Young's rule and 237.7 mg by body surface area — a 58.5% spread. That disagreement is the finding: children are not small adults, and published paediatric dosing should be used instead.
Formula
Three scaling rules, three answers. Published mg/kg dosing from paediatric studies is the modern standard.
Worked Example
- Clark's rule. (21 ÷ 70) × 500 = 150.0 mg.
- Young's rule. [6 ÷ (6 + 12)] × 500 = 166.7 mg — note this used no weight.
- Body surface area. √(116 × 21 ÷ 3,600) = 0.8226 m², so (0.8226 ÷ 1.73) × 500 = 237.7 mg.
- Measure the disagreement. 150.0 to 237.7 mg — a 58.5% spread for one child.
- Use the published dose. 15 mg/kg × 21 kg = 315 mg, from studies in children rather than scaling an adult.
Notice that the published paediatric dose of 315 mg sits ABOVE all three scaled estimates. That is not a contradiction — it is the point. Children often clear drugs faster per kilogram than adults do, because clearance tracks metabolic rate and surface area rather than mass, so scaling an adult dose downwards can leave a child underdosed just as easily as it can overdose them. Young's rule is the clearest failure: it uses no weight at all, so a 16 kg and a 28 kg six-year-old receive identical amounts. These rules are taught for historical context and for examination questions; they are not how children are dosed, and a calculator that presents one of them as the answer is doing harm.
Strengths & Limits Of This Model
Where this engine is strong
- Shows all three historical rules together so the disagreement is visible.
- Places the published mg/kg dose as the headline rather than a scaled estimate.
- States explicitly where scaling fails worst — infants and children off the centiles.
Where it stops
- Cannot supply the published paediatric dose; you must enter it.
- Cannot account for drug-specific paediatric pharmacokinetics.
Practical Use Cases
Understanding why scaling rules are taught but not used
The 58.5% spread between them is the reason.
Checking a published paediatric dose against weight
Enter the mg/kg from the formulary; that figure is the one to use.
Oncology and narrow-therapeutic-index drugs
BSA remains standard there. See the Body Surface Area Calculator.
Converting a dose to a measurable volume
Once the mg is settled, use the Dosage Calculator.
Methodology & Editorial Standards
This engine computes the three historical paediatric scaling rules alongside a published weight-based dose, and presents their disagreement as the primary finding rather than offering any one of them as the answer. Clark's rule scales the adult dose by the ratio of the child's weight to a nominal 70 kilogram adult; Young's rule scales by a function of age alone and therefore ignores body size entirely; the body surface area method scales by the ratio of the child's Mosteller surface area to a nominal 1.73 square metres. The three routinely differ by more than half for the same child, because they scale by different physical properties and clearance does not track any of them universally. Modern practice uses milligram-per-kilogram dosing established by pharmacokinetic studies conducted in children, with surface area dosing retained for oncology and narrow-therapeutic-index agents where the relationship to metabolic rate is closer. The engine identifies the specific circumstances in which scaling fails most severely, namely infants and neonates whose renal and hepatic clearance matures week by week, and children above the weight centiles where total body weight overstates the requirement for water-soluble drugs. Because paediatric doses are small, the engine emphasises the decimal-point error. This calculator is an educational estimate and is not medical advice, diagnosis, treatment or a prescription. It performs arithmetic on values you enter and cannot assess your clinical condition. Never start, stop, change or skip a dose of any medicine on the basis of a number produced here. Dose selection belongs to a prescriber working from the approved product label, your full medication list and your current clinical state. If you are acutely unwell, dehydrated, vomiting, or your kidney function is changing, estimating equations are unreliable and urgent clinical review is required. The engine implements the standard published formula for this calculation. Inputs are validated for domain and sign before evaluation, and any undefined case returns an em-dash rather than a spurious value.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Pediatric Dose Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is Clark's rule?
Child's weight in kg divided by 70, times the adult dose. It assumes a child is a linearly scaled adult.
What is Young's rule?
Age divided by age plus twelve, times the adult dose. It uses no weight, which is its fatal flaw.
Which paediatric rule is most accurate?
Of the three, body surface area. But none should replace a published mg/kg dose from paediatric studies.
Why do the rules disagree so much?
They scale by different properties — weight, age and surface area. Here they span 58.5%.
Are these rules still used?
Rarely, and only when no paediatric dose exists. They are taught mainly for historical and examination purposes.
Why is BSA better?
Metabolic rate and many clearance processes track surface area rather than mass, especially in small children.
What is the Mosteller formula?
BSA = √(height in cm × weight in kg ÷ 3,600). It is the standard simple BSA estimate.
Can I use these for a baby?
No. Under one year clearance changes week by week. Neonatal dosing requires a specialist formulary.
Why is the published dose higher than the scaled ones?
Children often clear drugs faster per kilogram, so scaling down from an adult can underdose them.
What about an overweight child?
Total body weight overstates the dose for water-soluble drugs. Ideal or adjusted weight may be used instead.
Should I ever exceed the adult dose?
Essentially never. A paediatric dose approaching the adult amount should be treated as an error until verified.
What is the commonest paediatric dosing error?
A misplaced decimal point — a tenfold error, and children have far less physiological reserve.
Does age matter at all?
Yes, but through organ maturity rather than as a scaling factor. That is why age-based rules fail.
Where do I find the published dose?
A paediatric formulary or the product label. A pharmacist is the right person to ask.
Do I round paediatric doses?
To a measurable volume, yes, but conservatively. Small absolute errors matter far more in children.
Can this tool prescribe for my child?
No. It illustrates why scaling rules disagree. Dosing a child is a prescriber's decision.
Is this pediatric dose calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.