A medication is ordered at 5 mg/kg per dose. If the patient weighs 18 kg, what is the dose in mg?

A medication is ordered at 5 mg/kg per dose. If the patient weighs 18 kg, what is the dose in mg?

• A. 90 mg
• B. 108 mg
• C. 180 mg
• D. 45 mg

Answer: (A)

Dosing by weight uses a simple multiply-and-unit-cancel rule. Take the amount given per kilogram and multiply by the patient’s weight in kilograms: 5 mg/kg × 18 kg. Multiply 5 by 18 to get 90, and the units work out to milligrams. So the dose per administration is 90 mg. If instead you used a different per-kilogram dose, you’d get different totals: 108 mg would come from 6 mg/kg, 180 mg from 10 mg/kg, and 45 mg from 2.5 mg/kg.

Dosing by weight uses a simple multiply-and-unit-cancel rule. Take the amount given per kilogram and multiply by the patient’s weight in kilograms: 5 mg/kg × 18 kg. Multiply 5 by 18 to get 90, and the units work out to milligrams. So the dose per administration is 90 mg.

If instead you used a different per-kilogram dose, you’d get different totals: 108 mg would come from 6 mg/kg, 180 mg from 10 mg/kg, and 45 mg from 2.5 mg/kg.