Paeds · clinical-pharmacology-and-therapeutics
Weight-based dosing, body-surface area and dose calculation
Also known as Paediatric weight-based drug dosing · mg per kilogram dosing in children · Body-surface area dosing and the Mosteller formula · Maximum adult dose cap in paediatric prescribing · Therapeutic drug monitoring and dose calculation
A fellowship approach to calculating a safe paediatric drug dose: weigh the child in kilograms on a calibrated scale, measure length or height for body-surface-area drugs, take the mg per kg or Mosteller body-surface-area dose from a current paediatric formulary, calculate the dose, cap it at the maximum adult single and daily dose, round sensibly, write a leading zero, and have a second clinician check it independently. For therapeutic drug monitoring drugs such as vancomycin and gentamicin, sample at the correct time and adjust to the AUC or trough target. Never estimate a weight unless it is an emergency, and never let a tenfold decimal error reach the child.
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- A tenfold error is the classic paediatric dosing disaster: a missing or misplaced decimal turns a correct dose into ten times too much or ten times too little, and it is caught only by an independent double check and a maximum adult dose cap.
- Always write a leading zero before a decimal (write 0.5 mg, never .5 mg) and never add a trailing zero after a whole number (write 5 mg, never 5.0 mg), because both invite a tenfold overdose.
- The paediatric dose calculated by weight must never exceed the adult maximum single dose or the adult maximum daily dose: calculate by weight, then cap at the adult ceiling.
- Record and prescribe every weight in kilograms, never pounds, and never mix units: a pound weight mistaken for kilograms inflates a dose by a factor of about 2.2.
- For therapeutic drug monitoring drugs, sample the level at the correct time relative to the dose: a vancomycin trough drawn too early overestimates the level and a gentamicin random level without a recorded dose time is uninterpretable.
- In the overweight or obese child, the wrong weight scalar for a narrow-therapeutic-index drug causes toxicity or failure: choose total, ideal or adjusted body weight deliberately, not by habit.
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- Clinical Sciences — Pharmacology
- Clinical Sciences — Pharmacology 10–12
- Current 2026 PREP curriculum — Learning Objective 1.5.2: Demonstrate safe and quality use of medicines, including accurate dose calculation and monitoring
- Current 2026 PREP curriculum — Learning Objective 2.2.1: Recognise, prioritise and manage an acutely ill infant, child or young person
- Renewed curriculum for first-year trainees from 2027 — Learning goal 4: Safe and effective prescribing
- Clinical Applications
- Medical Sciences
- Long Cases
- Short Cases
- 7. Patient safety, including safe prescribing
- 4. Professional skills and knowledge: Patient management
- 8. Pharmacology: Lead on the safe and effective use of medicines
- Applied Knowledge in Practice (AKP)
- Theory and Science (TAS)
- Clinical
- History
- Communication
- General Pediatrics Content Outline — Universal Task 4: Management and Treatment
- General Pediatrics Content Outline — Universal Task 7: Patient Safety and Quality Improvement
- General Pediatrics EPA 4: Writing Prescriptions
- Patient Care 5: Patient Management
- Systems-Based Practice 1: Patient Safety
- Practice-Based Learning 2: Reflects on unsafe practice and medication safety
- Medical Expert
- Pediatrics: Foundations EPA #5 — Prescribing medications safely
- Pharmacotherapeutic stewardship and safe medication use
Overview & Definition
Weight-based dosing is the practice of sizing a child's medicine to the body, because an adult dose given to a small child is a poison and an infant dose given to an adolescent is a failure. The child is weighed in kilograms on a calibrated scale, the dose per kilogram is taken from a current paediatric formulary, and the two are multiplied; the result is then capped at the maximum adult dose so a large adolescent never receives more than an adult would. Body-surface-area dosing extends the same idea to drugs whose clearance tracks metabolic mass rather than weight alone, and it is calculated from the Mosteller formula at the bedside. The discipline lives in the details: the weight must be right, the units must be kilograms, the decimal point must be in the right place, and a second person must check the answer. [2] [12]
A mg per kg dose multiplies a weight in kilograms by a milligram-per-kilogram figure to give the dose for one administration, and the result is never allowed to exceed the adult maximum single dose. A body-surface-area dose multiplies a body-surface area in square metres by a milligram-per-square-metre figure, and is reserved for drugs whose distribution and clearance track metabolic body size, such as cytotoxic chemotherapy, some antivirals and some antibiotics. A flat or fixed dose gives the same amount to every patient regardless of size and is used for a small number of medicines where the therapeutic window is wide and the pharmacokinetics are forgiving. The paediatric prescriber chooses the right scalar deliberately, because using the wrong scalar for a narrow-therapeutic-index drug is a leading cause of harm. [1] [10]
The unifying principle is safety by design: a calculation that is easy to get wrong is made safer by a measured weight, a standard formula, a cap, a rounding rule, and an independent check. Children are not small adults because their body composition, their protein binding, their liver enzyme maturation and their renal clearance change with age, so a single adult dose divided by some fraction of body weight is never a safe rule. The exam answer is the workflow, not the arithmetic of one drug: weigh, measure, look up, calculate, cap, round, write, check. [2] [5]
References12ShowHide
- [1]El Edelbi R, Lindemalm S, Eksborg S Estimation of body surface area in various childhood ages--validation of the Mosteller formula Acta Paediatrica, 2012.PMID 22211780
- [2]Hirata KM, Kang AH, Ramirez GV, et al. Pediatric Weight Errors and Resultant Medication Dosing Errors in the Emergency Department Pediatric Emergency Care, 2019.PMID 28976456
- [3]Lesar TS Tenfold medication dose prescribing errors Annals of Pharmacotherapy, 2002.PMID 12452740
- [4]Procaccini D, Kim JM, Lobner K, et al. Medication Errors in Overweight and Obese Pediatric Patients: A Narrative Review Joint Commission Journal on Quality and Patient Safety, 2022.PMID 35045950
- [5]Kendrick JG, Carr RR, Ensom MH Pediatric Obesity: Pharmacokinetics and Implications for Drug Dosing Clinical Therapeutics, 2015.PMID 26361823
- [6]Kyler KE, Wagner J, Hosey-Cojocari C, et al. Drug Dose Selection in Pediatric Obesity: Available Information for the Most Commonly Prescribed Drugs to Children Paediatric Drugs, 2019.PMID 31432433
- [7]McNeil JC, Kaplan SL Vancomycin Therapeutic Drug Monitoring in Children: New Recommendations, Similar Challenges Journal of Pediatric Pharmacology and Therapeutics, 2020.PMID 32839650
- [8]Rybak MJ, Le J, Lodise TP, et al. Therapeutic Monitoring of Vancomycin for Serious Methicillin-resistant Staphylococcus aureus Infections: A Revised Consensus Guideline and Review by the American Society of Health-system Pharmacists, the Infectious Diseases Society of America, the Pediatric Infectious Diseases Society, and the Society of Infectious Diseases Pharmacists Clinical Infectious Diseases, 2020.PMID 32658968
- [9]Hodiamont CJ, van den Broek AK, de Vroom SL, et al. Clinical Pharmacokinetics of Gentamicin in Various Patient Populations and Consequences for Optimal Dosing for Gram-Negative Infections: An Updated Review Clinical Pharmacokinetics, 2022.PMID 35754071
- [10]Demirhan S, Munoz FM, Valencia Deray KG, et al. Body surface area compared to body weight dosing of valganciclovir is associated with increased toxicity in pediatric solid organ transplantation recipients American Journal of Transplantation, 2023.PMID 37499799
- [11]Stevens JV, Prieto NS, Ridelman E, et al. Weight-based vs body surface area-based fluid resuscitation predictions in pediatric burn patients Burns, 2023.PMID 35351355
- [12]Carroll AR, Johnson JA, Stassun JC, et al. Health Literacy-Informed Communication to Reduce Discharge Medication Errors in Hospitalized Children: A Randomized Clinical Trial JAMA Network Open, 2024.PMID 38227315