To calculate a prescribed dose using a stock medication, which sequence of steps is correct?

• A. Multiply the desired dose by the stock quantity, divide by the stock strength, then round if needed and verify units.
• B. Multiply the stock strength by the stock quantity, then divide by the desired dose.
• C. Add the stock quantity to the stock strength, then divide by the desired dose.
• D. Subtract the stock strength from the stock quantity, then multiply by the desired dose.

Answer: (A)

The key idea is using a proportion to convert the prescribed dose into the amount of stock to dispense. You want to end up with the number of dosage units (tablets, mL, etc.) that will supply the prescribed amount, based on what you have on hand and the drug’s potency per unit.

In this approach, you multiply the desired dose by the stock quantity and then divide by the stock strength. This sequence effectively scales the prescribed amount to the total stock you have and then converts that scaled amount into the number of stock units needed. Because multiplication and division can be rearranged, this is equivalent to (desired dose ÷ stock strength) × stock quantity, which is the standard way to convert a dose into the count of stock units.

After doing the calculation, you round if needed and verify the units to ensure you’re giving the correct form (for example, the right number of tablets or the correct milliliters) and that the amount is within what is available.