Most pediatric doses are the child’s weight times mg per kg
Weight-based dosing is the standard for children because drug handling tracks body mass. Take the child’s weight in kilograms and multiply by the drug’s per-kilogram dose. To convert from pounds, divide by 2.205. Every result is still bounded by a minimum and the adult maximum.
The age-based rules, and what fraction of the adult dose they give
When weight isn’t known, older rules estimate from age. They are approximate and considered less reliable than mg/kg dosing; Young’s applies to ages 1 to 12, Fried’s to infants under 1.
| Rule | Formula | Example fraction of adult dose |
|---|---|---|
| Clark’s (weight) | lb ÷ 150 × adult dose | 66 lb child → 44% |
| Young’s (age 1–12) | age ÷ (age + 12) × adult dose | age 4 → 25%; age 12 → 50% |
| Fried’s (under 1) | months ÷ 150 × adult dose | 6 months → 4% |
These give the same dose to two children of the same age or weight regardless of size, which is why weight-based mg/kg dosing is preferred wherever the weight is known.
Why the estimate needs a professional check
- Children aren’t small adults. Immature liver and kidney function in infants means drugs clear more slowly, so the youngest patients often need the most conservative dosing.
- Caps matter both ways. A dose is held to the adult maximum at the top and a safe minimum at the bottom; the raw calculation is a starting point, not the final number.
- Concentration errors are common. The same liquid medicine comes in different strengths, so the volume to give depends on the bottle. Verify the calculated dose and the concentration with a pharmacist before giving anything.
Common questions
How is a weight-based pediatric dose calculated?
Multiply the child's weight in kilograms by the drug's dose in mg per kg. A drug dosed at 25 mg/kg given to a 20 kg child works out to 500 mg per dose. This is the standard method for most pediatric medicines.
What is Clark's rule and when is it used?
Clark's rule scales the adult dose by weight: child weight in pounds divided by 150, times the adult dose. It is a legacy method, used only when a proper weight-based dose isn't available, and it's less reliable than dosing by mg/kg.
Can a calculated dose ever be higher than the adult dose?
The math can produce one for a large adolescent, but the dose is normally capped at the adult maximum. A dose should never exceed the adult limit, and never fall below the safe minimum, whatever the calculation says.
