When infusing with a drop factor, which variables are used to calculate drops per minute?

• A. Time only
• B. Volume, drop factor, and time
• C. Drop factor only
• D. Volume only

Answer: (B)

The calculation uses three pieces of information: the total volume to be infused (in milliliters), the drop factor of the IV set (drops per milliliter), and the infusion time (in minutes). These three are combined in the formula: drops per minute = (volume in mL) × (drop factor in gtt/mL) ÷ (time in minutes). The drop factor tells you how many drops make up one milliliter, so multiplying by the volume converts to drops, and dividing by the time gives the rate in drops per minute. For example, 1000 mL over 480 minutes with a 15 gtt/mL set yields about 31 drops per minute. Without any one of these three pieces, you can’t determine the correct infusion rate.