Clark's Rule Pediatric Dosage Calculator
Why pediatric dosing needs extra caution
Important: This calculator provides an educational estimate using an old “rule of thumb” (Clark’s Rule). It is not a prescription tool and should not replace instructions from a clinician, pharmacist, or the medication label. Pediatric medication dosing can be high-risk because children’s drug absorption, distribution, metabolism, and elimination change with age, organ maturity, and illness—factors that a simple weight ratio cannot capture.
Clark’s Rule is still commonly discussed because it’s quick and easy when you only have a listed adult dose and the child’s weight. Modern practice, however, usually relies on drug-specific pediatric guidance (often mg/kg, sometimes body surface area, and frequently with maximum single-dose and daily limits). Use the output here as a starting point for understanding—and then verify against trusted pediatric references.
What Clark’s Rule is (and what it assumes)
Clark’s Rule estimates a child’s dose by scaling an adult dose according to the child’s weight as a fraction of a reference adult weight of 150 lb. The classic version uses pounds. This page accepts kilograms and converts internally to pounds so you can enter weight in the units most commonly used in clinical settings outside the U.S.
Formula (plain text + MathML)
Plain text:
Child dose (mg) = (Child weight (lb) / 150) × Adult dose (mg)
If you enter kilograms:
Child dose (mg) = ((Child weight (kg) × 2.20462) / 150) × Adult dose (mg)
MathML:
Where Dc is the estimated child dose, Wlb is the child’s weight in pounds, and Da is the standard adult dose (in mg).
How to use the calculator (units & input tips)
- Child Weight: enter the child’s current weight in kg. If you only know pounds, convert first (lb ÷ 2.20462 = kg) or use a reliable conversion tool.
- Standard Adult Dose: enter a single adult dose amount in mg (for example, 400 mg per dose). If the adult dose is given as a range (e.g., 200–400 mg), calculate both ends and treat the result as an estimate range—then confirm against labeling.
- Result: the output is in mg per dose based on the adult dose you entered. It does not determine dosing frequency (e.g., every 6 hours) or daily maximums.
Interpreting the result safely
The number you see is a proportional estimate. Real-world dosing decisions often require additional constraints:
- Maximum single dose / maximum daily dose: many drugs cap pediatric doses even if mg/kg scaling would suggest more.
- Formulation constraints: tablets, liquids (mg/mL), chewables, extended-release products, and measuring devices can make certain doses impractical or unsafe to approximate.
- Indication and age band: some medications have different dosing by condition, not just weight; infants and toddlers may have separate guidance.
- Renal/hepatic impairment and interactions: these can substantially change safe dosing and are not reflected here.
If you are a caregiver and you are unsure, the safest next step is to check the product’s pediatric label or ask a pharmacist—especially for prescription drugs, concentrated liquids, or any medicine with narrow safety margins.
Worked example
Scenario: Adult dose is 400 mg. Child weighs 30 kg.
- Convert weight to pounds:
30 kg × 2.20462 ≈ 66.14 lb - Compute the weight fraction:
66.14 / 150 ≈ 0.4409 - Scale the adult dose:
0.4409 × 400 mg ≈ 176.36 mg
Estimated child dose: about 176 mg (often rounded for practicality—only if doing so remains consistent with labeling and maximum-dose limits).
Clark’s Rule compared with other pediatric dose estimation methods
Clark’s Rule is only one historical estimation approach. Others exist and may be more appropriate depending on the medication and data available:
| Method | Typical formula | Strengths | Limitations |
|---|---|---|---|
| Clark’s Rule (weight-based) | (Weightlb / 150) × Adult dose | Very simple; only requires weight | Assumes linear scaling and a 150-lb “standard” adult; ignores age/organ maturity |
| Young’s Rule (age-based) | (Age / (Age + 12)) × Adult dose | Simple when weight is unavailable | Age alone is a weak proxy; rarely preferred in modern dosing |
| Body Surface Area (BSA) | (BSA / 1.73) × Adult dose | Often correlates better with physiology for some drugs | Requires height and weight; still not a substitute for drug labeling |
| Drug-specific mg/kg dosing (modern labeling) | (mg/kg) × Weightkg | Based on clinical evidence and pediatric studies | Must follow indication-specific guidance and max dose rules |
Limitations & assumptions (read before using)
- Educational estimate only: Not medical advice; do not use as the sole basis for dosing.
- Linear scaling assumption: Assumes dose scales directly with body weight, which is not reliably true across ages and medications.
- Reference adult weight is fixed at 150 lb: This historical constant may not match many real adults and may bias estimates up or down.
- Not suitable for infants/neonates: Very young children can have markedly different pharmacokinetics; many medications have special neonatal/infant dosing.
- No maximum dose safeguards: The calculator does not apply maximum single-dose or daily maximum limits.
- No frequency guidance: It estimates an amount per dose based on what you input; it does not determine how often to give the medication.
- Formulation and concentration not considered: mg-to-mL conversions for liquids and device accuracy are not handled here.
- Clinical factors ignored: Kidney/liver disease, drug interactions, dehydration, obesity, and severity of illness can change dosing decisions.
Practical FAQs
Why does Clark’s Rule use 150 lb?
It is a historical “average adult” reference used in the original rule. Modern adult averages vary by population; the constant is kept mainly for consistency with the traditional method.
What if the adult dose is listed as a range?
Compute the child estimate for both the low and high adult doses to create an estimate range, then confirm the correct pediatric dose using labeling or a pharmacist.
Does this apply to all medications?
No. Some medicines are dosed by mg/kg with strict maximums; others are not weight-linear at all. Always defer to drug-specific pediatric recommendations.