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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 example

A 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.

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Weight-Based Dosing
A pediatric patient weighs 82.5 lb. The order is gentamicin 7.5 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.
mg/dose

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10 Weight-Based (mg/kg) Dosing practice problems with worked solutions

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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
  1. Convert: 17.6 lb ÷ 2.2 lb/kg = 8 kg.
  2. Daily dose: 60 mg/kg/day × 8 kg = 480 mg/day.
  3. Per dose: 480 mg/day ÷ 4 doses/day = 120 mg/dose.
  4. 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
  1. Convert: 60.5 lb ÷ 2.2 lb/kg = 27.5 kg.
  2. Daily dose: 25 mg/kg/day × 27.5 kg = 687.5 mg/day.
  3. Per dose: 687.5 mg/day ÷ 2 doses/day ≈ 343.8 mg/dose.
  4. 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
  1. Convert: 22 lb ÷ 2.2 lb/kg = 10 kg.
  2. Daily dose: 40 mg/kg/day × 10 kg = 400 mg/day.
  3. Per dose: 400 mg/day ÷ 4 doses/day = 100 mg/dose.
  4. 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
  1. Convert: 44 lb ÷ 2.2 lb/kg = 20 kg.
  2. Daily dose: 60 mg/kg/day × 20 kg = 1200 mg/day.
  3. Per dose: 1200 mg/day ÷ 4 doses/day = 300 mg/dose.
  4. 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
  1. Convert: 11 lb ÷ 2.2 lb/kg = 5 kg.
  2. Daily dose: 40 mg/kg/day × 5 kg = 200 mg/day.
  3. Per dose: 200 mg/day ÷ 4 doses/day = 50 mg/dose.
  4. 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
  1. Convert: 82.5 lb ÷ 2.2 lb/kg = 37.5 kg.
  2. Daily dose: 40 mg/kg/day × 37.5 kg = 1500 mg/day.
  3. Per dose: 1500 mg/day ÷ 3 doses/day = 500 mg/dose.
  4. 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
  1. Convert: 11 lb ÷ 2.2 lb/kg = 5 kg.
  2. Daily dose: 80 mg/kg/day × 5 kg = 400 mg/day.
  3. Per dose: 400 mg/day ÷ 3 doses/day ≈ 133.3 mg/dose.
  4. 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
  1. Convert: 22 lb ÷ 2.2 lb/kg = 10 kg.
  2. Daily dose: 5 mg/kg/day × 10 kg = 50 mg/day.
  3. Per dose: 50 mg/day ÷ 3 doses/day ≈ 16.7 mg/dose.
  4. 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
  1. Convert: 77 lb ÷ 2.2 lb/kg = 35 kg.
  2. Daily dose: 10 mg/kg/day × 35 kg = 350 mg/day.
  3. Per dose: 350 mg/day ÷ 1 doses/day = 350 mg/dose.
  4. 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
  1. Convert: 30.8 lb ÷ 2.2 lb/kg = 14 kg.
  2. Daily dose: 10 mg/kg/day × 14 kg = 140 mg/day.
  3. Per dose: 140 mg/day ÷ 4 doses/day = 35 mg/dose.
  4. 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.

Sources and how far they go

Where a convention on this page comes from a published standard, the standard is named. Where it comes from a textbook or from ordinary practice instead, the entry says so. That difference matters when a preceptor or an exam disagrees with you.

Citations are to the source of the method. They are not a review of this page, and none of these bodies has reviewed or endorsed it.

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