IV Drip Rate (Drops per Minute)

Before pumps, and still whenever a pump is unavailable, the flow is set by counting drops in the drip chamber against a watch. The tubing manufacturer prints a drop factor on the packet: how many drops make a millilitre. Macrodrip sets are 10, 15 or 20 gtt/mL; microdrip, or paediatric, sets are 60. A litre through a 15 gtt/mL set over eight hours needs $1000 \ imes 15 / 480 = 31.25$ drops a minute, which you would count as about 31.

The microdrip set hides a small piece of arithmetic worth committing to memory. With 60 drops to the millilitre and 60 minutes to the hour, the two sixties cancel and the drops per minute equal the millilitres per hour exactly. A patient on 200 mL/h through a 60 gtt/mL set is running at 200 gtt/min, no calculation required. That identity is why paediatric sets are made at 60 in the first place.

The classic error is using the wrong drop factor, and it is unforgiving: the same bag and the same time on a 10 gtt/mL set versus a 60 gtt/mL set differ sixfold. Read the packet, not the memory. And understand what gravity drip actually is, which is a flow set by hydrostatic head and the resistance of a roller clamp. Bag height, patient position, a partially positional cannula and the falling head as the bag empties all change the rate over the hours, so a gravity line is checked and reset repeatedly rather than calculated once.