Which formula is correct for calculating drops per minute given volume, drop factor, and time?

• A. Volume × Drop factor ÷ Time (minutes)
• B. Volume ÷ (Drop factor × Time)
• C. Time × Drop factor ÷ Volume
• D. Volume × Time ÷ Drop factor

Answer: (A)

Delivering IV fluids by the correct rate means turning the volume to be infused (in mL) into a number of drops per minute, using the drop factor (drips per mL). The drop factor tells you how many drops make up 1 mL. To get the rate in drops per minute, you first convert the volume to drops by multiplying by the drop factor, then divide by the time in minutes to convert the total drops into a per-minute rate. That gives volume × drop factor ÷ time.

For example, infusing 100 mL over 30 minutes with a 15 gtt/mL set: (100 × 15) ÷ 30 = 1500 ÷ 30 = 50 drops per minute.

Other forms don’t yield a rate in drops per minute because their units don’t line up to give drops per minute. They would mix the quantities in a way that doesn’t represent a flow rate.