60. A microdrip set allows 60 gtt (drops) per ML.
The drop factor differs (# of gtts/mL). A set that delivers large drops per mL (10-20 gtt/ml, usually 15) is called macrodrip set. One uses smaller gtts (60gtt/mL) is called microdrip. Usually the macrodrip set is used if the IV rate is to infuse > 100ml/hr. Vice versa, Microdrip < 100ml/hr volume. Here is the drip rate calculation: Volume (mL)/ time (min) x drip factor= gtt/min ranges 30-100 gtt/min.
The volume of a drop can vary depending on the liquid in question. Surface tension, the speed the drop is excreted and temperature will all subtly affect volume. However the average volume is usually 40 to 50 micro litres per drop.