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
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.