Infusion rate & drop factor

Convert between infusion rate in mL/hr, drops per minute, and total infusion time.

What is this for?

When an infusion is regulated by a roller clamp rather than a pump, the rate is set in drops per minute. The conversion depends on the tubing's drop factor — how many drops make up a millilitre — which is printed on the set packaging.

How to use it

Select the drop factor printed on the administration set.
Enter any two of total volume, rate in mL/hr, and infusion time in hours.
Read the drops per minute. The seconds-between-drops figure is useful for setting the rate by watch.

Worked example

Infuse 1 litre over 8 hours using tubing with a drop factor of 20 gtt/mL. What rate in mL/hr, and how many drops per minute?

Answer: 1000 ÷ 8 = 125 mL/hr. Drops: 20 × 125 ÷ 60 = 41.7, so set about 42 drops per minute — roughly one drop every 1.4 seconds.

Clinical pearls & pitfalls

The drop factor is a property of the tubing, not of the fluid or the pump. Changing the administration set changes the drops-per-minute answer for the same mL/hr.
Microdrip tubing at 60 gtt/mL has a convenient property: drops per minute equals mL per hour exactly, because the 60 in the drop factor cancels the 60 minutes.
Round drops per minute to a whole number — you cannot count a fraction of a drop. Do not round the mL/hr rate, which is the clinically meaningful quantity.
Gravity infusions drift as the bag empties and the patient moves. Recheck the rate periodically; for anything with a narrow therapeutic index, use a pump.

Assumptions & limitations

Assumes a constant gravity-driven rate. Real drip rates vary with bag height, patient position, and cannula patency.
Drop size varies slightly with fluid viscosity and surface tension, so the nominal drop factor is itself an approximation.
Not suitable for infusions requiring precise control — vasopressors, insulin, chemotherapy, paediatric fluids — which require a volumetric pump.

References

Gorski LA, et al. Infusion Therapy Standards of Practice, 8th edition. J Infus Nurs. 2021;44(1S):S1-S224.
Institute for Safe Medication Practices. Guidelines for Optimizing Safe Implementation and Use of Smart Infusion Pumps.