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MedTestBankDosage Calculation for Nursing

Weight-Based Dosage Calculator (mg/kg)

Calculate doses ordered in mg/kg, mcg/kg or units/kg, per dose or per day divided, with pounds to kilograms conversion and the volume to give.

mL

Enter the values: the answer and every step appear here as you type.

Formula

Weight (kg) = weight (lb) ÷ 2.2

Dose = dose per kg × weight in kg

Dose per dose = daily dose ÷ number of doses per day

How to calculate it step by step

  1. Convert the weight to kilograms if it is in pounds (divide by 2.2) and round to the tenth.
  2. Multiply the ordered dose per kg by the weight in kg.
  3. If the order is per day, divide by the number of doses per day (every 8 hours = 3 doses, every 6 hours = 4 doses).
  4. If needed, convert the dose to a volume with desired ÷ have × quantity.
  5. Compare the result with the safe range and the maximum dose in a drug reference.

Worked examples

Example 1. Order: ceftriaxone 50 mg/kg IV once daily. The child weighs 45 lb. How many mg will you give? Round the weight in kg to the tenth first.

  1. Convert: 45 lb ÷ 2.2 = 20.5 kg.
  2. Dose = 50 mg/kg × 20.5 kg = 1,025 mg, rounded to 1,025 mg.

Answer: 1,025 mg

Example 2. Order: ceftriaxone 50 mg/kg IV once daily. The child weighs 24.3 kg. How many mg will you give?

  1. Dose = 50 mg/kg × 24.3 kg = 1,215 mg, rounded to 1,215 mg.

Answer: 1,215 mg

Rounding

Round the weight to the tenth of a kg before calculating unless your program says otherwise. Round mg doses to the tenth, unit doses to the whole unit, and volumes as for any liquid.

Common mistakes

  • Multiplying by the weight in pounds: always convert to kg first.
  • Confusing mg/kg/dose with mg/kg/day: a daily dose must be divided by the number of doses.
  • Giving more than the adult maximum dose to a heavy child or adult: weight-based doses often have a cap.

Nursing safety points

  • Use a weight measured in kg on admission, not an estimated or stated weight, especially for children.
  • Pediatric doses are checked against the safe range for every order; contact the prescriber if the dose is outside it.
  • Document the weight used for the calculation.

Frequently asked questions

How do you calculate mg/kg?

Multiply the dose per kg by the weight in kg. A 44 lb child is 44 ÷ 2.2 = 20 kg; at 15 mg/kg the dose is 15 × 20 = 300 mg.

How do you convert pounds to kilograms?

Divide by 2.2. For example, 154 lb ÷ 2.2 = 70 kg.

What is the difference between mg/kg/dose and mg/kg/day?

mg/kg/dose is the amount for each dose. mg/kg/day is the total for 24 hours, which you divide by the number of doses ordered.