What is the basic formula for calculating a weight-based medication dose (mg/kg) for a patient?

• A. Dose (mg) = weight (kg) ÷ prescribed dose per kilogram (mg/kg)
• B. Dose (mg) = weight (lb) × prescribed dose per kilogram (mg/kg)
• C. Dose (mg) = weight (kg) × prescribed dose per kilogram (mg/kg)
• D. Dose (mg) = weight (kg) + prescribed dose per kilogram (mg/kg)

Answer: (C)

When a dose is given per kilogram, you determine the total amount to give by multiplying the patient’s weight in kilograms by the prescribed dose per kilogram. This works because mg/kg multiplied by kg cancels the unit kg and leaves mg, giving the total dose in milligrams.

For example, if a patient weighs 20 kg and the dose is 5 mg/kg, the total dose is 20 × 5 = 100 mg.

Division or addition would not produce the correct unit or amount. Dividing weight by mg/kg would yield an incomprehensible unit, using pounds without converting to kilograms ignores the per-kilogram basis, and adding weight to a per-kilogram dose mixes incompatible units. If weight is provided in pounds, convert to kilograms first (kg = lb ÷ 2.2046) before applying the formula.