Which expression best describes the calculation for IV drip rate when using volume, time, and a drop factor?

• A. (Volume to be infused over one hour) ÷ Drop factor
• B. (Volume to be infused) ÷ (Time in minutes) × Drop factor
• C. Drop factor ÷ Volume to be infused
• D. Volume to be infused × Time ÷ Drop factor

Answer: (B)

Calculating IV drip rate uses volume to be infused, time, and drop factor to determine how many drops per minute should be delivered. The goal is to deliver the specified volume over the given time, and the drop factor tells you how many drops make up each milliliter.

The expression that gives the correct rate is to multiply the volume by the drop factor and then divide by the time. Since (volume × drop factor) ÷ time is the same as (volume ÷ time) × drop factor, this form yields the drip rate in drops per minute. The units work out cleanly: volume in milliliters divided by time in minutes gives milliliters per minute, and multiplying by the drop factor (drops per milliliter) converts that to drops per minute.

For example, delivering 500 mL over 240 minutes with a drop factor of 20 gtt/mL: (500 ÷ 240) × 20 ≈ 41.7 gtt/min. This aligns with the expected rate, whereas the other forms produce unusable units or incorrect relationships between volume, time, and drops.