How this instrument works
Clark's Rule scales the adult dose by a child's weight against a 150 lb reference adult: child dose = adult dose × weight (lb) ÷ 150. It's attributed to Cecil Belfield Clarke, a Barbadian-born physician who practiced medicine in London for decades in the early-to-mid 20th century. Young's Rule instead scales by age alone: child dose = adult dose × age (years) ÷ (age + 12) — useful in the rare case where a child's weight isn't available but age is. Both take a single known adult dose and stretch it down using one simple ratio.
Both rules predate the era of precise, drug-specific weight-based (mg/kg) dosing data that now exists for most common pediatric medications. Wherever a drug-specific mg/kg dose is known — as it is for essentially every other calculator in this Health category, including acetaminophen, ibuprofen, and amoxicillin pediatric dosing — that direct weight-based figure is preferred and more accurate, because it's derived from actual pharmacokinetic data in children rather than a fixed ratio to an adult dose. Clark's Rule and Young's Rule exist as a fallback for situations where no such drug-specific figure is available, not as a first choice.
Because the two rules use completely different inputs, they can disagree noticeably for the same child, as the worked example below shows. Neither rule should ever be used for narrow-therapeutic-index drugs — chemotherapy agents, anticoagulants, digoxin, and similar medications — where a modest dosing error can cause serious harm and only a drug-specific, weight- or body-surface-area-based regimen supervised by a clinician is appropriate.
- Enter the "Standard adult dose (mg)" for the medication in question.
- Enter the child's weight in pounds under "Child's weight (lb)".
- Enter the child's age in whole years under "Child's age (years)".
- Compare the two outputs: "Clark's Rule estimate (mg)" (weight-based) and "Young's Rule estimate (mg)" (age-based) — the further apart they are, the less either should be trusted on its own.
Worked example — 500 mg adult dose, two children
A 6-year-old weighing 40 lb, with a standard adult dose of 500 mg: Clark's Rule gives 500 × 40 ÷ 150 ≈ 133.3 mg, based purely on weight relative to a 150 lb reference adult. Young's Rule, based purely on age, gives 500 × 6 ÷ (6 + 12) ≈ 166.7 mg for the same child and the same adult dose — noticeably higher than the weight-based figure.
A larger, older child — 60 lb and 10 years old, same 500 mg adult dose — doesn't close that gap: Clark's Rule gives 500 × 60 ÷ 150 = 200 mg, while Young's Rule gives 500 × 10 ÷ 22 ≈ 227.3 mg. How closely the two rules agree depends entirely on whether a particular child's age and weight happen to track the reference assumptions each formula was built on — there's no guarantee they will.
Questions
Why do Clark's Rule and Young's Rule sometimes give noticeably different answers for the same child?
Because they use entirely different inputs. Clark's Rule only looks at weight relative to a 150 lb reference adult; Young's Rule only looks at age relative to an age+12 scaling constant. A child who is heavier or lighter than average for their age will get estimates that pull apart, sometimes by a wide margin, since neither formula knows anything about the other's input.
Should I use Clark's Rule or Young's Rule instead of a mg/kg dose from a pediatrician, pharmacist, or drug label?
No. Wherever a drug-specific weight-based (mg/kg) dose is known, that figure is preferred and more accurate — it comes from actual pharmacokinetic study in children, while Clark's Rule and Young's Rule are just fixed ratios applied to an adult dose. These two rules are a fallback for when no such drug-specific figure exists, not a replacement for one that does.
Are these rules safe to use for chemotherapy, blood thinners, digoxin, or similar drugs?
No. Never use Clark's Rule or Young's Rule for narrow-therapeutic-index drugs, where the gap between an effective dose and a harmful one is small. Those medications require drug-specific, clinician-supervised dosing based on actual pharmacokinetic data — a generic ratio-to-adult-dose approximation is not precise enough to be safe.
Where does the 150 lb figure in Clark's Rule come from?
It's a reference average adult body weight that Cecil Belfield Clarke's formula scales the adult dose against. It's a fixed constant, not tailored to any particular drug or to how children's bodies actually process medication differently from adults'.
Why does Young's Rule divide by age + 12 instead of some other number?
It's a historical scaling constant built into the original formula so that the fraction (age ÷ (age + 12)) approaches 1 — the full adult dose — only as age grows very large, while producing smaller fractions for younger children. It isn't derived from any specific drug's pharmacokinetics; it's a generic age-based curve.
How old are these formulas, and why are they still taught?
Both predate modern pediatric clinical pharmacology by decades — Clark's Rule is attributed to Cecil Belfield Clarke, a Barbadian-born physician practicing in London in the early-to-mid 20th century, and Young's Rule is of similar vintage. They're still taught in nursing and pharmacy coursework as historical examples of dose-estimation logic and as a fallback method, even though direct mg/kg dosing built from real pharmacokinetic study in children — which became far more available through the 20th century — is now preferred wherever it exists.
Is this calculator a substitute for advice from a pharmacist or pediatrician?
No. It performs the arithmetic behind two historical estimation rules; it doesn't know what drug is involved, whether a more accurate mg/kg dose already exists for it, or anything about the specific child's health. Confirm any dose with a pharmacist or pediatrician before giving a medication.
References
- Clark's Rule — StatPearls, NCBI Bookshelf (NBK541104)
- Young's Rule — StatPearls, NCBI Bookshelf (NBK554603)
- Kearns GL, Abdel-Rahman SM, Alander SW, Blowey DL, Leeder JS, Kauffman RE — Developmental Pharmacology: Drug Disposition, Action, and Therapy in Infants and Children, N Engl J Med, 2003
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.