Weight-based dosage calculations (mg/kg)
Many pediatric orders are written in mg/kg/day. Convert the weight to kilograms, find the daily dose, then divide by the number of doses.
Method
- kg = lb ÷ 2.2.
- Daily dose = kg × mg/kg/day.
- Per-dose amount = daily dose ÷ doses per day (q12h = 2, q8h = 3, q6h = 4).
Worked examples
Example 1
A patient weighs 22 lb. The order is 40 mg/kg/day divided every 8 hours. How many mg per dose? Round to one decimal place.
Show solution
- Weight in kg = 22 lb ÷ 2.2 = 10 kg
- Daily dose = 10 kg × 40 mg/kg = 400 mg/day
- Per dose = 400 ÷ 3 = 133.3333 mg, which rounds to 133.3 mg
Answer: 133.3 mg
Example 2
A patient weighs 22 lb. The order is 50 mg/kg/day divided every 12 hours. How many mg per dose? Round to one decimal place.
Show solution
- Weight in kg = 22 lb ÷ 2.2 = 10 kg
- Daily dose = 10 kg × 50 mg/kg = 500 mg/day
- Per dose = 500 ÷ 2 = 250 mg
Answer: 250 mg
Example 3
A patient weighs 88 lb. The order is 40 mg/kg/day divided every 8 hours. How many mg per dose? Round to one decimal place.
Show solution
- Weight in kg = 88 lb ÷ 2.2 = 40 kg
- Daily dose = 40 kg × 40 mg/kg = 1,600 mg/day
- Per dose = 1,600 ÷ 3 = 533.3333 mg, which rounds to 533.3 mg
Answer: 533.3 mg
Tips
- Round only at the end to avoid compounding rounding error.
- These are practice exercises. Real dosing is verified by a pharmacist.
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