In more detail
The drop factor is printed on the administration set's packaging. Macrodrip sets commonly deliver 10, 15 or 20 drops per mL and microdrip sets 60 drops per mL; with a 60-drop set, drops per minute equal millilitres per hour. Gravity drips are counted against a watch and rounded to whole drops, while electronic infusion pumps are programmed in mL per hour instead. The basic formula is drops/min = volume (mL) × drop factor / time (min).
Why it matters
Using the wrong drop factor changes the dose: a rate worked out for a 15-drop set but run through a 20-drop set delivers a quarter less fluid. Gravity rates also drift as the bag empties or the patient moves, so they are re-checked regularly. In practice infusions are set and monitored by trained clinical staff following local protocols; this page explains the arithmetic only.
Example
1,000 mL over 8 h, 15 drops/mL: 1,000 × 15 / 480 = 31.25 → 31 drops/min (125 mL/h)
500 mL over 4 h, 20 drops/mL: 500 × 20 / 240 = 41.7 → 42 drops/min (125 mL/h)
Microdrip, 60 drops/mL, 125 mL/h: 125 × 60 / 60 = 125 drops/min
31.25 drops/min through a 20-drop set: 31.25 / 20 × 60 = 93.75 mL/h, not 125