AkCalculators

Pediatric Dosage — Principles, Rules, and Safe Calculation

Calculating accurate medication doses for children is one of the most important — and potentially dangerous — clinical tasks. Children are not “small adults.” Their pharmacokinetics (absorption, distribution, metabolism, elimination) differ across ages, and dosing must take weight, age, organ function, and drug-specific properties into account. The aim of this article is to explain common pediatric dosing principles, classic bedside rules (Clark, Young, Fried), when to use weight-based mg/kg dosing, rounding and formulation considerations, and safety checks every clinician or caregiver should perform.

Why weight matters

Most pediatric doses are expressed per kilogram of body weight (mg/kg) because body mass correlates with volume of distribution and clearance for many drugs. Weight-based dosing scales dosing to the child’s size rather than using a simple fraction of adult dose. This reduces the risk of underdosing or overdosing across a range of child sizes.

Common classic rules (Clark, Young, Fried)

Before weight was routinely measured at bedside, several heuristic rules were used to estimate pediatric doses from adult doses. These are still taught and occasionally used when weight is unavailable, but they are approximations and should be replaced by mg/kg dosing whenever possible.

• Clark’s rule — based on weight in pounds:
  Child's dose = (weight in lb ÷ 150) × adult dose.
  Clark’s rule assumes 150 lb as a standard adult weight. Another common variant uses 70 kg as the adult reference: (weight kg ÷ 70 kg) × adult dose. Clark’s rule is more accurate the closer the adult reference weight reflects the prescribing adult population.
• Young’s rule — based on age in years:
  Child's dose = (age ÷ (age + 12)) × adult dose.
  Young’s rule performs poorly for very small children because it assumes a linear relationship with age; it is mainly historical.
• Fried’s rule — for infants (in months):
  Infant's dose = (age in months ÷ 150) × adult dose.
  Fried’s rule is a simple scaling for infants, but it is a crude estimate and not appropriate for drugs with narrow therapeutic windows.

Weight-based dosing (mg/kg)

Whenever possible, calculate pediatric doses using the drug’s recommended mg/kg dosing. For example, an analgesic might be dosed 10 mg/kg every 6–8 hours (maximum 40 mg/kg/day). The calculation is straightforward:

Child dose (mg) = mg/kg × weight (kg)

Always check maximum single dose and maximum daily dose for the specific drug. For many drugs, manufacturers list both mg/kg dosing and recommended maximum absolute doses for safety.

Unit conversions and rounding

Common pitfalls are unit errors (mixing lb and kg) and excessive precision. Convert units before computing (1 lb = 0.45359237 kg). Round doses to clinically usable values: for liquids, round to the nearest 0.1 mL if the syringe allows it; for tablets, round to the nearest fraction of a tablet, but never recommend splitting controlled-release formulations unless specified. When volume is computed using drug concentration (mg/mL), ensure the resulting mL is practical for administration.

Calculating volume from concentration

If you know the drug concentration (mg per mL), computing the volume is:

Volume (mL) = Dose (mg) ÷ Concentration (mg/mL)

Round volume sensibly given the available delivery device. For neonates, small differences in volume matter — use appropriately sized syringes (0.01–0.1 mL precision) and consult pharmacy if volumes are extremely small.

Maximum doses and frequency

Many drugs have maximum single doses (e.g., paracetamol 1000 mg per dose) and maximum daily doses (e.g., paracetamol 60 mg/kg/day up to an absolute max). Include frequency (every 6–8 hours) when planning therapy and ensure cumulative daily dosing does not exceed safe limits. If the calculated mg/kg dose would exceed the recommended absolute maximum, use the lower absolute max instead.

Special populations

Preterm neonates, infants, and children with renal or hepatic impairment often need reduced doses or extended dosing intervals. Drug clearance in neonates may be markedly reduced; consult neonatal dosing references for neonates and preterms. Similarly, certain chemotherapy or narrow-therapeutic-index drugs require weight-and-surface-area-based dosing (mg/m²) and specialist oversight.

Examples

Example A — mg/kg dosing: Paracetamol 15 mg/kg per dose for a child 12 kg → 15 × 12 = 180 mg per dose. If syrup concentration is 160 mg/5 mL (32 mg/mL), volume = 180 ÷ 32 ≈ 5.6 mL.

Example B — Clark’s rule: Adult dose 500 mg, child weight 33 lb → (33 ÷ 150) × 500 = 0.22 × 500 = 110 mg (approx).

Practical checklist before administering a pediatric dose

• Confirm child’s current weight in kilograms — weigh when possible.
• Confirm age (prematurity adjustments if neonate/preterm).
• Confirm correct drug, concentration, and route.
• Compute mg/kg and compare to recommended range and absolute max.
• Round dose to a practical unit and document calculation.
• When in doubt, consult a pediatric pharmacist or prescribing information.

Limitations of heuristic rules

Clark’s, Young’s, and Fried’s rules are approximations and were developed when bedside weight measurement and standardized references were less available. They can be helpful in emergencies when weight is unknown, but modern practice favors weight-based dosing and consultation with pharmacy for unusual drugs or critically ill children.

Conclusion

Safe pediatric dosing begins with an accurate weight and uses mg/kg dosing tailored to the drug’s recommended range and maximums. Classic rules (Clark, Young, Fried) can provide quick estimates but should not replace mg/kg calculations. Always confirm doses against up-to-date references, check maximum single and daily doses, and involve a pharmacist for complex or high-risk medications.

Disclaimer: This page provides educational calculations only. Always verify pediatric doses with a qualified clinician or pharmacist before administration.