Randomized mg/kg problems that build in the steps exams like to stack: a weight in pounds, a daily dose that has to be divided, and a liquid that has to be drawn up. Easy is a single multiplication. Hard is all three.
Convert the weight to kilograms (pounds ÷ 2.2), multiply by the ordered mg/kg, and divide by the number of doses if the order is per day. A 33 lb child ordered 30 mg/kg/day divided every 8 hours: 33 ÷ 2.2 = 15 kg; 15 × 30 = 450 mg/day; 450 ÷ 3 = 150 mg per dose.
Read the order twice for "per dose" or "per day". 10 mg/kg every 8 hours and 30 mg/kg/day divided every 8 hours give the same answer, but 30 mg/kg every 8 hours gives three times as much. That single word is the most expensive one in pediatric math.
| Order | Setup | Answer |
|---|---|---|
| 5 mg/kg; 70 kg | 70 × 5 | 350 mg |
| 10 mg/kg; 44 lb; 100 mg/5 mL | 44 ÷ 2.2 = 20 kg; 20 × 10 = 200 mg; 200 ÷ 100 × 5 | 200 mg = 10 mL |
| 30 mg/kg/day ÷ q8h; 33 lb; 250 mg/5 mL | 15 kg × 30 = 450 mg/day; ÷ 3 = 150 mg; 150 ÷ 250 × 5 | 150 mg = 3 mL |
| 40 mg/kg/day ÷ q12h; 25 kg | 25 × 40 = 1,000 mg/day; ÷ 2 | 500 mg per dose |
| Frequency | Doses per day |
|---|---|
| Every 4 hours (q4h) | 6 |
| Every 6 hours (q6h) | 4 |
| Every 8 hours (q8h) | 3 |
| Every 12 hours (q12h) | 2 |
| Every 24 hours (q24h) | 1 |
Divide 24 by the interval. If an exam gives "three times a day" instead of q8h, it still means three doses.
Nursing exams use 2.2 lb per kg. The exact figure is about 2.2046, which is why your answer may differ slightly from a calculator that converts more precisely. Use 2.2 unless your program says otherwise.
Programs differ on when to round the weight. Some round kilograms to the tenth before using them; others carry the full number through and round only the final dose. The two can differ in the last decimal place, so the practice tool accepts either path and shows you which one it used. In practice, weight is measured in kilograms on the unit scale and documented in kilograms, which removes the conversion and its errors altogether.
The classic mistake is using the number in pounds as if it were kilograms. A 44 lb child treated as 44 kg gets more than double the dose. If a weight-based answer looks large for the patient in the question, check the conversion first.
Convert the weight to kilograms, then multiply by the ordered mg/kg. If the order is per day, divide by the number of doses. 15 kg at 30 mg/kg/day divided every 8 hours is 450 mg/day, or 150 mg per dose.
Nursing programs and exams use 2.2. The exact factor is about 2.2046, which can shift an answer slightly in the last decimal place. Use 2.2 unless your program says otherwise.
Follow your program. Some round kilograms to the tenth first, others carry the full value and round only at the end. The practice tool accepts either and explains the difference.
Calculate the total daily dose from the weight, then split it into three equal doses, one every 8 hours. 40 mg/kg/day for 25 kg is 1,000 mg/day; divided q12h that is 500 mg per dose.