Weight-Based (mg/kg) Dosing Practice
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Many drugs, especially in pediatrics, are dosed by body weight in mg/kg. Two things decide whether you get these right: converting pounds to kilograms, and telling a total daily dose apart from a per-dose amount.
Formulakg = lb ÷ 2.2 • daily dose = mg/kg/day × weight • per dose = daily dose ÷ number of doses
Worked exampleA 66 lb child on amoxicillin 45 mg/kg/day divided twice daily. Kg = 66 ÷ 2.2 = 30 kg; daily = 45 × 30 = 1,350 mg; per dose = 1,350 ÷ 2 = 675 mg.
mg/kg/DAY vs mg/kg/DOSE is the trap that catches most people here. Read which one the order specifies.
How to set up a weight-based dose
Pediatric doses, and many adult doses of antibiotics, anticoagulants and chemotherapy, are written per kilogram. The calculation is a multiplication, and the two places it goes wrong are before and after it: converting the weight, and reading whether the order is a daily total or a single dose.
kg = lb ÷ 2.2 • daily dose = mg/kg/day × kg • per dose = daily dose ÷ doses per day
Worked example: mg/kg/day divided
A child weighs 44 lb. The order is amoxicillin 45 mg/kg/day divided twice daily. How many mg per dose?
weight = 44 ÷ 2.2 = 20 kg
daily dose = 45 × 20 = 900 mg/day
per dose = 900 ÷ 2 = 450 mg
If the drug is a suspension, one more step turns the dose into a volume. With amoxicillin 400 mg/5 mL:
volume per dose = 450 ÷ 400 × 5 = 5.625 mL
which the pharmacist rounds to a volume the dispensed syringe can measure. The practice problems on this site stop at the mg per dose; the volume step belongs to the days-supply and dispensing-quantity topics.
mg/kg/day versus mg/kg/dose
The same numbers read the other way produce a very different dose. Acetaminophen 40 mg/kg/day in four divided doses for a 20 kg child is 800 mg a day and 200 mg a dose. Misread as 40 mg/kg/dose, it becomes 800 mg per dose, four times a day: 3,200 mg a day to a 20 kg child. The words "per day" and "divided" in an order mean a daily total that you split. "Per dose" or a frequency written after the mg/kg (10 mg/kg every 6 hours) means each dose is that size and the daily total is the multiplication, not the division.
When the order gives a range (25 to 50 mg/kg/day), a question will tell you which end to use, or ask for the maximum. Compute both ends if it isn't clear and read the stem again.
Which weight
Weight-based orders for children use actual weight, and that's what the problems here assume. Some adult drugs specify ideal or adjusted body weight instead, and the question will say so when it matters. If it gives a height alongside the weight for an adult, that's a hint that IBW or AdjBW is expected; the creatinine clearance topic covers how to calculate those.
Where students lose the points
- Pounds used as kilograms. The dose comes out 2.2 times too large. Convert first and write "kg" next to the converted number.
- Multiplying by 2.2 instead of dividing. A kilogram is heavier than a pound, so a weight in kg is always the smaller number.
- Daily total reported as the dose. Divide by the number of doses when the order says "divided" or "per day".
- Dividing a per-dose order. 10 mg/kg per dose is not divided further.
- 2.2 vs 2.2046. Use 2.2 unless the question says otherwise. The difference is under a quarter of a percent, but it can move a rounded answer by a tenth.
10 Weight-Based (mg/kg) Dosing practice problems with worked solutions
Every problem below is worked start to finish. Try it yourself first, then check the steps. For a fresh problem every time, use the practice box above. It generates unlimited variations of these same question types.
Problem 1: Weight-Based Dosing
A pediatric patient weighs 17.6 lb. The order is vancomycin 60 mg/kg/day divided four times daily (q6h). How many mg per dose? (Use 1 kg = 2.2 lb.) Round your answer to one decimal place.
Answer 120.0 mg/dose
Solution- Convert: 17.6 lb ÷ 2.2 lb/kg = 8 kg.
- Daily dose: 60 mg/kg/day × 8 kg = 480 mg/day.
- Per dose: 480 mg/day ÷ 4 doses/day = 120 mg/dose.
- Rounded as the question asks: 120.0 mg/dose.
Watch for mg/kg/DAY vs mg/kg/DOSE, the trap that catches most people in this family. Conversion factors also vary: 2.2 lb/kg is the exam convention, 2.2046 is the exact figure, so the question states which to use.
Problem 2: Weight-Based Dosing
A pediatric patient weighs 60.5 lb. The order is cephalexin 25 mg/kg/day divided twice daily (q12h). How many mg per dose? (Use 1 kg = 2.2 lb.) Round your answer to one decimal place.
Answer 343.8 mg/dose
Solution- Convert: 60.5 lb ÷ 2.2 lb/kg = 27.5 kg.
- Daily dose: 25 mg/kg/day × 27.5 kg = 687.5 mg/day.
- Per dose: 687.5 mg/day ÷ 2 doses/day ≈ 343.8 mg/dose.
- Rounded as the question asks: 343.8 mg/dose.
Watch for mg/kg/DAY vs mg/kg/DOSE, the trap that catches most people in this family. Conversion factors also vary: 2.2 lb/kg is the exam convention, 2.2046 is the exact figure, so the question states which to use.
Problem 3: Weight-Based Dosing
A pediatric patient weighs 22 lb. The order is acetaminophen 40 mg/kg/day divided four times daily (q6h). How many mg per dose? (Use 1 kg = 2.2 lb.) Round your answer to one decimal place.
Answer 100.0 mg/dose
Solution- Convert: 22 lb ÷ 2.2 lb/kg = 10 kg.
- Daily dose: 40 mg/kg/day × 10 kg = 400 mg/day.
- Per dose: 400 mg/day ÷ 4 doses/day = 100 mg/dose.
- Rounded as the question asks: 100.0 mg/dose.
Watch for mg/kg/DAY vs mg/kg/DOSE, the trap that catches most people in this family. Conversion factors also vary: 2.2 lb/kg is the exam convention, 2.2046 is the exact figure, so the question states which to use.
Problem 4: Weight-Based Dosing
A pediatric patient weighs 44 lb. The order is acetaminophen 60 mg/kg/day divided four times daily (q6h). How many mg per dose? (Use 1 kg = 2.2 lb.) Round your answer to one decimal place.
Answer 300.0 mg/dose
Solution- Convert: 44 lb ÷ 2.2 lb/kg = 20 kg.
- Daily dose: 60 mg/kg/day × 20 kg = 1200 mg/day.
- Per dose: 1200 mg/day ÷ 4 doses/day = 300 mg/dose.
- Rounded as the question asks: 300.0 mg/dose.
Watch for mg/kg/DAY vs mg/kg/DOSE, the trap that catches most people in this family. Conversion factors also vary: 2.2 lb/kg is the exam convention, 2.2046 is the exact figure, so the question states which to use.
Problem 5: Weight-Based Dosing
A pediatric patient weighs 11 lb. The order is vancomycin 40 mg/kg/day divided four times daily (q6h). How many mg per dose? (Use 1 kg = 2.2 lb.) Round your answer to one decimal place.
Answer 50.0 mg/dose
Solution- Convert: 11 lb ÷ 2.2 lb/kg = 5 kg.
- Daily dose: 40 mg/kg/day × 5 kg = 200 mg/day.
- Per dose: 200 mg/day ÷ 4 doses/day = 50 mg/dose.
- Rounded as the question asks: 50.0 mg/dose.
Watch for mg/kg/DAY vs mg/kg/DOSE, the trap that catches most people in this family. Conversion factors also vary: 2.2 lb/kg is the exam convention, 2.2046 is the exact figure, so the question states which to use.
Problem 6: Weight-Based Dosing
A pediatric patient weighs 82.5 lb. The order is vancomycin 40 mg/kg/day divided three times daily (q8h). How many mg per dose? (Use 1 kg = 2.2 lb.) Round your answer to one decimal place.
Answer 500.0 mg/dose
Solution- Convert: 82.5 lb ÷ 2.2 lb/kg = 37.5 kg.
- Daily dose: 40 mg/kg/day × 37.5 kg = 1500 mg/day.
- Per dose: 1500 mg/day ÷ 3 doses/day = 500 mg/dose.
- Rounded as the question asks: 500.0 mg/dose.
Watch for mg/kg/DAY vs mg/kg/DOSE, the trap that catches most people in this family. Conversion factors also vary: 2.2 lb/kg is the exam convention, 2.2046 is the exact figure, so the question states which to use.
Problem 7: Weight-Based Dosing
A pediatric patient weighs 11 lb. The order is amoxicillin 80 mg/kg/day divided three times daily (q8h). How many mg per dose? (Use 1 kg = 2.2 lb.) Round your answer to one decimal place.
Answer 133.3 mg/dose
Solution- Convert: 11 lb ÷ 2.2 lb/kg = 5 kg.
- Daily dose: 80 mg/kg/day × 5 kg = 400 mg/day.
- Per dose: 400 mg/day ÷ 3 doses/day ≈ 133.3 mg/dose.
- Rounded as the question asks: 133.3 mg/dose.
Watch for mg/kg/DAY vs mg/kg/DOSE, the trap that catches most people in this family. Conversion factors also vary: 2.2 lb/kg is the exam convention, 2.2046 is the exact figure, so the question states which to use.
Problem 8: Weight-Based Dosing
A pediatric patient weighs 22 lb. The order is gentamicin 5 mg/kg/day divided three times daily (q8h). How many mg per dose? (Use 1 kg = 2.2 lb.) Round your answer to one decimal place.
Answer 16.7 mg/dose
Solution- Convert: 22 lb ÷ 2.2 lb/kg = 10 kg.
- Daily dose: 5 mg/kg/day × 10 kg = 50 mg/day.
- Per dose: 50 mg/day ÷ 3 doses/day ≈ 16.7 mg/dose.
- Rounded as the question asks: 16.7 mg/dose.
Watch for mg/kg/DAY vs mg/kg/DOSE, the trap that catches most people in this family. Conversion factors also vary: 2.2 lb/kg is the exam convention, 2.2046 is the exact figure, so the question states which to use.
Problem 9: Weight-Based Dosing
A pediatric patient weighs 77 lb. The order is azithromycin 10 mg/kg/day divided once daily. How many mg per dose? (Use 1 kg = 2.2 lb.) Round your answer to one decimal place.
Answer 350.0 mg/dose
Solution- Convert: 77 lb ÷ 2.2 lb/kg = 35 kg.
- Daily dose: 10 mg/kg/day × 35 kg = 350 mg/day.
- Per dose: 350 mg/day ÷ 1 doses/day = 350 mg/dose.
- Rounded as the question asks: 350.0 mg/dose.
Watch for mg/kg/DAY vs mg/kg/DOSE, the trap that catches most people in this family. Conversion factors also vary: 2.2 lb/kg is the exam convention, 2.2046 is the exact figure, so the question states which to use.
Problem 10: Weight-Based Dosing
A pediatric patient weighs 30.8 lb. The order is clindamycin 10 mg/kg/day divided four times daily (q6h). How many mg per dose? (Use 1 kg = 2.2 lb.) Round your answer to one decimal place.
Answer 35.0 mg/dose
Solution- Convert: 30.8 lb ÷ 2.2 lb/kg = 14 kg.
- Daily dose: 10 mg/kg/day × 14 kg = 140 mg/day.
- Per dose: 140 mg/day ÷ 4 doses/day = 35 mg/dose.
- Rounded as the question asks: 35.0 mg/dose.
Watch for mg/kg/DAY vs mg/kg/DOSE, the trap that catches most people in this family. Conversion factors also vary: 2.2 lb/kg is the exam convention, 2.2046 is the exact figure, so the question states which to use.