π Pediatric Dose Calculator
Clark and Young rules for child dosing, explained.
For informational purposes only, not medical advice. Consult a healthcare professional.
Before weight-based dosing became standard, clinicians scaled adult doses down to children with two rules of thumb. Clark rule scales by weight: child dose equals adult dose times weight in pounds divided by 150. Young rule scales by age: adult dose times age divided by age plus 12. Both are rough, both are still taught, and both are inferior to proper mg-per-kg dosing from a pediatric reference. This calculator runs both rules for education and double-checking, with a clear warning that real dosing comes from pediatric references and pharmacists.
How to Use This Calculator
Enter the standard adult dose, the child weight, and the child age. The calculator applies both classic rules so you can compare them side by side.
Enter the adult dose exactly as the label states it, in milligrams. Both rules scale from this number.
Enter weight in kilograms and age in years. Clark rule uses weight, Young rule uses age, so both inputs get used.
Compare the two results. Modern practice prefers true weight-based mg-per-kg dosing from references; these rules are the historical backup when only an adult dose is known.
How the Math Works
Clark rule: multiply the adult dose by the child weight in pounds, then divide by 150, the reference adult weight in pounds the rule assumes. Young rule: multiply the adult dose by age in years, then divide by age plus 12. A 500 mg adult dose for a 20 kg (44 lb) 6-year-old gives about 147 mg by Clark and about 167 mg by Young. The rules often disagree, which is itself the lesson: they are approximations from an era before pediatric pharmacology matured. Current references dose most drugs directly in mg per kg, which is more accurate than either rule.
Worked Example
For a 500 mg adult dose, a 20 kg six-year-old gets 147.0 mg by Clark rule and 166.7 mg by Young rule. The two classic rules disagree by about 20 mg, which is normal and instructive: neither is exact. A pharmacist checking this prescription would reach for the pediatric reference, which for many drugs would suggest a weight-based dose in the same neighborhood but grounded in actual pediatric data.
Reading Your Result
When the two rules roughly agree, they corroborate each other as a sanity check on a reference dose. When they diverge widely, trust neither and look up the drug: narrow-therapeutic-index drugs like digoxin or chemotherapy agents must never be dosed by rule of thumb. For most common medicines, the pediatric label or a reference like the Harriet Lane Handbook gives direct mg-per-kg dosing that supersedes both rules. Use this calculator to understand the history and to double-check, not to originate a dose.
| Method | Basis | Best for |
|---|---|---|
| Clark rule | Weight vs 150 lb adult | Quick weight scaling |
| Young rule | Age vs age+12 | When weight unknown |
| mg per kg reference | Pediatric data | Actual prescribing |
Limitations
These rules ignore body composition, organ maturity, and drug-specific pharmacokinetics; neonates and infants metabolize drugs very differently from older children. They assume a 150-pound adult reference that fits few real adults. Never use either rule for high-risk drugs, and confirm every pediatric dose against a current reference or pharmacist. This is an educational tool, not a prescribing aid.
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For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is Clark rule?
Child dose equals the adult dose multiplied by the child weight in pounds divided by 150. It assumes a 150-pound reference adult and scales linearly by weight. Published in the early 1900s, it remains the most quoted of the historical dosing rules.
What is Young rule?
Child dose equals the adult dose multiplied by age in years divided by age plus 12. It needs only the age, which made it handy before scales were everywhere. Like Clark rule, it is an approximation, not a pharmacokinetic model.
Which rule is more accurate?
Usually Clark, because weight predicts drug handling better than age. But both lose to direct mg-per-kg dosing from a pediatric reference, which is based on actual studies in children. Think of the rules as arithmetic backup, not as dosing science.
Why not just use these rules for every drug?
Because children are not scaled-down adults. Liver enzymes mature at different rates, kidney function per kilogram is higher in toddlers, and body water percentages differ. Some drugs need higher per-kg doses in children than adults, which no simple scaling rule captures.
Do these rules work for babies?
Poorly. Neonates and young infants have immature metabolism and excretion, so scaled adult doses can dangerously overshoot. Infant dosing always needs pediatric-specific references. If the patient is under 2, these rules are the wrong tool entirely.
What is the 150 in Clark rule?
The assumed adult weight in pounds, about 68 kg. It was a reasonable average when the rule was devised. Modern adults vary widely around it, which is one more reason the rule is approximate.
Can I use these rules for over-the-counter medicines?
Most OTC children medicines now carry weight-based dosing charts on the label, which beat both rules. Use the label chart with an accurate weight. The rules are for the rare case where only an adult dose is known and a pharmacist is not at hand, and even then confirm afterward.
What is Fried rule?
A third historical rule for infants under 2: adult dose times age in months divided by 150. Like the others, it is obsolete for real prescribing but still appears in exams. The pattern is the same: simple scaling, no pharmacology.
Why do the two rules give different answers?
They scale on different inputs with different reference assumptions, so disagreement is expected, especially at the extremes of weight for age. A stocky 6-year-old and a slight 6-year-old get the same Young dose but different Clark doses. The divergence is a feature: it warns you the answer is approximate.
What should I actually use to dose a child?
A current pediatric drug reference or the pharmacist. Weight-based mg-per-kg dosing with attention to maximum doses, age limits, and the specific drug pharmacokinetics is the standard of care. These historical rules help you understand old prescriptions and pass exams; they should not write new ones.