Weight-based doses (per kilogram) and paediatric safety
- Calculation
Many doses, especially in children, are ordered per kilogram: multiply by the actual weight and check that the result is within a safe range.
What it is
In children, and for many adult drugs, the dose is ordered per kilogram of body weight (mg/kg or mcg/kg), per dose or per day. If it is per day, divide by the number of doses. The weight must be recent and in kilograms. Besides calculating the dose, the nurse checks that the ordered dose is within the safe range of the reference and does not exceed the maximum dose. The examples use fictional drugs and made-up numbers.
Formulas
- Dose (mg) = mg/kg × weight (kg).
- Dose per administration = daily dose ÷ doses per day (every 12 h = 2; every 8 h = 3; every 6 h = 4).
- Volume (mL) = dose (mg) ÷ strength (mg/mL), or dose ÷ H × Q.
- Safe range: minimum dose/kg × weight up to maximum dose/kg × weight.
How to calculate it
- Confirm the weight in kilograms and when it was measured — an old weight changes the dose.
- Multiply the dose per kilogram by the weight — gives the total dose.
- If the order is per day, divide by the number of doses — gives the dose per administration.
- Work out the safe range from the reference and compare — catches orders that are too high or too low.
- Convert the dose to a volume using the strength — this is what goes in the syringe.
- Ask for an independent double check — errors are frequent in children.
Worked examples (fictional drugs; educational, not a clinical order)
| Data | Set-up | Answer |
|---|---|---|
| Child, 18 kg. Order: drug A 15 mg/kg per dose. On hand: 120 mg/5 mL. | 15 × 18 = 270 mg; 270 ÷ 120 × 5 mL | 270 mg = 11.25 mL |
| Infant, 12.5 kg. Order: drug B 30 mg/kg/day divided every 8 h. On hand: 250 mg/5 mL. | 30 × 12.5 = 375 mg/day; 375 ÷ 3 = 125 mg; 125 ÷ 250 × 5 mL | 125 mg = 2.5 mL per dose |
| Girl, 22 kg. Safe range of drug C: 10–15 mg/kg per dose. Order: 400 mg. | 10 × 22 = 220 mg; 15 × 22 = 330 mg | 400 mg is above the range (220–330 mg): do not give, clarify |
| Adult, 70 kg. Order: drug D 0.5 mg/kg. On hand: 50 mg/mL. | 0.5 × 70 = 35 mg; 35 ÷ 50 × 1 mL | 35 mg = 0.7 mL |
| Newborn, 3.2 kg. Order: drug E 5 mcg/kg. On hand: 20 mcg/mL. | 5 × 3.2 = 16 mcg; 16 ÷ 20 × 1 mL | 16 mcg = 0.8 mL |
Common mistakes
- Using pounds as if they were kilograms.
- Forgetting to divide the daily dose by the number of doses (the whole day's dose is given at once).
- Not comparing with the safe range.
- Using an estimated or out-of-date weight.
Double-checking
- Calculate yourself and ask another person to calculate without seeing your answer.
- Compare the result with the drug's maximum dose in the institutional reference.
- If the volume is very small, use a syringe that can measure it exactly.
Test yourself
Think of the answer, then tap the question to see it.
If an order is in mg/kg/day and given every 8 hours, what is the daily dose divided by?
By 3, because every 8 hours means three doses a day.
What does the nurse do if the ordered dose is above the calculated safe range?
Does not give it and clarifies the order with the prescriber.
Educational example: 10 mg/kg for a 14 kg child. What is the dose?
10 × 14 = 140 mg (made-up numbers, educational, not a clinical order).
Connections
Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order.
Sources: Nursing Skills (Open RN), 5.11 Weight-Based Calculations (Open RN, Chippewa Valley Technical College (LibreTexts), 2022); Nursing Skills (Open RN), 5.12 Safe Dosage Range (Open RN, Chippewa Valley Technical College (LibreTexts), 2022); Nursing Skills (Open RN), 5.5 Rounding (Open RN, Chippewa Valley Technical College (LibreTexts), 2022).
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Representative model, for educational purposes only.