Which formula correctly calculates the volume needed when given the desired dose and the concentration?

• A. Volume = Dose × Concentration
• B. Dose = Concentration × Volume
• C. Volume = Dose ÷ Concentration
• D. Dose = Volume ÷ Concentration

Answer: (C)

The key idea is the relationship between dose, concentration, and volume. Dose equals concentration times volume. To get the volume when you know the dose and the concentration, solve for volume: Volume = Dose ÷ Concentration. This makes sense because concentration tells you how much drug is in each milliliter, so dividing the total amount you need (the dose) by that amount per milliliter will tell you how many milliliters to administer.

For example, if you need 50 mg and the solution has a concentration of 10 mg/mL, the volume needed is 50 mg ÷ 10 mg/mL = 5 mL.

Other forms don’t directly yield volume. Volume equals Dose times Concentration would mix units in a way that doesn’t give a volume. The standard, direct way to compute volume from dose and concentration is Volume = Dose ÷ Concentration.