For gravity infusions, multiply the volume by the tubing drop factor (gtt/mL) and divide by the time in minutes. Round to whole drops because you cannot count part of a drop.
What each part means
Meaning of each symbol in the IV drip rate (gtt/min) formula
Volume
Total volume to infuse (mL)
Drop factor
Drops per mL for the tubing (10, 15, 20, or 60)
Time
Infusion time in minutes
Worked example
1000 mL over 8 hr with 15 gtt/mL tubing: (1000 × 15) ÷ 480 = 31.25 ≈ 31 gtt/min.