🧮 Dosage Calculations · Lesson 4 of 5 · 6 min
mg per kg: Pediatric and Safe-Dose Checks
👶 Pediatric orders come with a safe range in mg per kg per day. The exam gives you the order, the weight and the range, and asks a question nobody expects: is this dose safe to give at all?
💡 Key idea
Kilograms first, then dose per kg times kg. For a range, compute the daily minimum and maximum, divide by doses per day, and hold anything outside it.
🧠 Why it matters
First convert the weight to kilograms. Then multiply: dose per kg × kg = dose. For a SAFE DOSE RANGE given per day, multiply both ends of the range by the weight to get the daily minimum and maximum, then divide by the number of doses per day to compare with the ordered single dose. Example: a 20 kg child is ordered 250 mg every 8 hours; the safe range is 30 to 50 mg/kg/day. Daily range: 600 to 1,000 mg; three doses a day means 200 to 333 mg per dose; 250 mg is within range, so it is safe to give. If the ordered dose falls outside the range, HOLD and clarify with the prescriber; do not adjust it yourself. For BODY SURFACE AREA dosing (chemotherapy), the dose is mg per square meter times the BSA from a chart or formula. For continuous weight-based infusions, such as mcg/kg/min, convert step by step: mcg per minute, then mcg per hour, then divide by the concentration to get mL/hr.
🏥 In the real world
🧮 An infant weighs 8 kg. Order: acetaminophen 160 mg every 6 hours. Safe range: 10 to 15 mg/kg/dose. 10 × 8 = 80 mg and 15 × 8 = 120 mg per dose. The ordered 160 mg exceeds the maximum. The nurse holds the dose and calls the prescriber. The calculation took 30 seconds; the liver it protected was the whole point.
📌 Takeaways
- Convert to kg before anything else
- Per-day ranges: multiply by weight, then divide by doses per day
- Outside the range: hold and clarify, never adjust on your own
✅ Test yourself
A child weighs 22 lb. The order is for 15 mg/kg of a medication. What is the correct dose?
- 150 mg
- 100 mg
- 66 mg
- 330 mg
Answer: A · 150 mg
22 ÷ 2.2 = 10 kg; 15 × 10 = 150 mg.
A 30 kg child is ordered 400 mg of an antibiotic every 8 hours. The safe range is 25 to 50 mg/kg/day. Is the dose safe?
- Yes, it is within the safe range
- It cannot be determined
- No, it exceeds the maximum daily dose
- No, it is below the minimum daily dose
Answer: A · Yes, it is within the safe range
Daily range: 750 to 1,500 mg. Ordered: 400 × 3 = 1,200 mg/day, which is within range.
The ordered dose for an infant falls above the safe range calculated by the nurse. What should the nurse do?
- Reduce the dose to the maximum safe amount
- Hold the dose and contact the prescriber for clarification
- Give half now and half later
- Give the dose as ordered because the prescriber is responsible
Answer: B · Hold the dose and contact the prescriber for clarification
Nurses do not change prescribed doses independently; unsafe orders are held and clarified.
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