For clinicians and students
Pediatric dosage calculations: formulas and rules
For clinicians and students. Not a substitute for your institution’s protocols.
Most pediatric doses today are weight-based: dose (mg) = mg/kg × weight in kg, and volume (mL) = dose ÷ concentration (mg/mL). If a reference gives mg/kg/day, divide by the number of doses per day. The older Clark’s, Young’s and Fried’s rules scale an adult dose by weight or age; they appear on exams but aren’t used for real prescribing.
Source: StatPearls Publishing
The formulas
| Method | Formula | Use |
|---|---|---|
| Weight-based (per dose) | dose = mg/kg × kg | Standard modern method |
| Weight-based (per day) | dose = mg/kg/day × kg ÷ doses per day | Standard modern method |
| Volume | mL = dose (mg) ÷ concentration (mg/mL) | Converting to a liquid |
| Body surface area | dose = mg/m² × BSA | Chemotherapy and some others |
| Young’s rule | adult dose × age ÷ (age + 12) | Historical, age 1-12 years |
| Clark’s rule | adult dose × weight (lb) ÷ 150 | Historical |
| Fried’s rule | adult dose × age (months) ÷ 150 | Historical, under 2 years |
Young’s rule: StatPearls Publishing. Clark’s and Fried’s rules are standard textbook formulas.
Compare the historical rules
For exam practice only. Never use these to set a real dose.
Worked example: mg/kg/day
Order: 40 mg/kg/day divided every 8 hours. Child: 44 lb. Stock: 250 mg/5 mL.
- Weight: 44 lb ÷ 2.2 = 20 kg.
- Daily dose: 40 × 20 = 800 mg/day.
- Per dose: every 8 hours is 3 doses, so 800 ÷ 3 = 266.7 mg.
- Volume: 250 mg/5 mL is 50 mg/mL, so 266.7 ÷ 50 = 5.3 mL per dose.
Try more in the dosage calculation practice, or check real prescriptions with the weight-based dose calculator.
Why the old rules fell out of use
They assume a child is a scaled-down adult. Children absorb, distribute and clear many drugs differently, and those differences don’t track age or weight in a straight line, especially in infancy. Drug-specific mg/kg or mg/m² doses from pediatric references replaced them.
Common questions
What is Young’s rule?
Child’s dose = adult dose × age in years ÷ (age + 12). It’s a historical rule based on age alone.
What is Clark’s rule?
Child’s dose = adult dose × weight in pounds ÷ 150. It scales the adult dose to a 150 lb adult.
What is Fried’s rule?
For infants under 2: child’s dose = adult dose × age in months ÷ 150.
Are these rules still used?
Mostly for exams. Modern pediatric dosing uses mg/kg (or mg/m² of body surface area) from drug references, because age- and weight-ratio rules ignore how children handle drugs differently from adults.
Sources
- Young’s Rule (Wade, Martinez). StatPearls Publishing, 2023 (NBK554603). Checked 2026-09-25.
- Simplified calculation of body-surface area (Mosteller). N Engl J Med 1987;317(17):1098. Checked 2026-09-25.