When using a drop factor to calculate IV rate in drops per minute, which formula is correct?

• A. Rate (mL/hour) = total volume ÷ time
• B. Rate (mL/min) = (volume ÷ time) × drop factor
• C. Rate (drops/min) = (volume × drop factor) ÷ time
• D. Rate (drops/hour) = volume ÷ time × drop factor

Answer: (C)

Using the drop factor converts milliliters to drops, so to get a rate in drops per minute you convert the volume to drops and divide by the infusion time in minutes. Multiply the volume (in mL) by the drop factor (drops per mL) to get total drops, then divide by the time (minutes) to spread those drops over each minute: drops per minute = (volume × drop factor) ÷ time. For example, infusing 50 mL over 30 minutes with a drop factor of 15 drops/mL gives (50 × 15) ÷ 30 = 25 drops per minute. This directly yields the required unit and uses the drop factor correctly. The other forms either mislabel the units, omit the drop factor, or yield a different time-based unit (drops per hour) rather than drops per minute.