🧠 The theory: two calculations joined together
A paediatric question is usually a dosing calculation and a liquid-dose calculation stitched together. First the child's weight sets the dose (mg/kg), then the suspension's concentration turns that dose into a volume. Splitting it into those two familiar halves, dose first and volume second, is what makes an intimidating question routine. The extra care is in the safety checks, because children have far less margin for error.
Worked example
A 20 kg child needs cefalexin 25 mg/kg/day in two divided doses. The suspension is 125 mg in 5 mL. What volume is given per dose?
- Step 1 — Total daily dose from the weight: 25 x 20 = 500 mg a day.
- Step 2 — Split across the two doses: 500 / 2 = 250 mg per dose.
- Step 3 — Concentration of the suspension: 125 / 5 = 25 mg/mL.
- Step 4 — Volume for the dose: 250 / 25 = 10.
Answer: 10 mL per dose
Worked example
A 15 kg child needs amoxicillin 20 mg/kg per dose. The suspension is 250 mg in 5 mL. What volume per dose?
- Step 1 — Dose from the weight: 20 x 15 = 300 mg.
- Step 2 — Concentration: 250 / 5 = 50 mg/mL.
- Step 3 — Volume: 300 / 50 = 6.
Answer: 6 mL per dose
Worked example
A 12 kg child needs paracetamol 15 mg/kg per dose. The suspension is 120 mg in 5 mL. What volume per dose?
- Step 1 — Dose: 15 x 12 = 180 mg.
- Step 2 — Concentration: 120 / 5 = 24 mg/mL.
- Step 3 — Volume: 180 / 24 = 7.5.
Answer: 7.5 mL per dose
Harder: maximum dose checks
Worked example
A 30 kg child is prescribed a drug at 10 mg/kg per dose, but the adult maximum is 250 mg. What dose is given?
- Step 1 — Calculate the weight-based dose: 10 x 30 = 300 mg.
- Step 2 — Compare with the adult maximum: 300 mg is above 250 mg.
- Step 3 — A child's dose must not exceed the adult maximum, so cap it at 250 mg.
Answer: 250 mg
⚠️ Always apply the safety checks
Check the calculated dose against the maximum for the child's age or weight, and never let a weight-based dose exceed the adult dose. Round to a volume that can actually be measured with an oral syringe, and use the BNF for Children as the reference.
Test yourself
An 18 kg child needs 5 mg/kg per dose. The suspension is 50 mg in 5 mL. What volume per dose?
Answer: Dose = 90 mg; concentration 10 mg/mL; volume = 90 / 10 = 9 mL
An 8 kg child needs 30 mg/kg/day in three divided doses. What is each dose?
Answer: Daily = 30 x 8 = 240 mg; per dose = 240 / 3 = 80 mg
A 14 kg child needs 30 mg/kg/day in three doses. The suspension is 125 mg in 5 mL. What volume per dose?
Answer: Daily = 420 mg; per dose = 140 mg; concentration 25 mg/mL; volume = 140 / 25 = 5.6 mL