IV drip rate formula: how to work out drops per minute

Drops per minute = volume (mL) × drop factor (gtt/mL) ÷ time (min).

The drop factor is the number of drops in one milliliter of solution through a particular gravity tubing set, and it is printed on the tubing package. You need it because a gravity infusion is set by counting drops in the drip chamber, not by a pump.

Most IV medications in the United States run through an infusion pump, which is programmed in milliliters per hour. Drip rates still come up on dosage calculation exams, and on the unit when an infusion has to run by gravity.

Macrodrip tubing delivers larger drops: Open RN lists drop factors of 10, 12, 15 or 20 drops per milliliter. Microdrip tubing delivers 60 drops per milliliter. Read the factor off the package in the problem instead of assuming one, because the same order gives a different drip rate for each drop factor.

Microdrip has a convenient property. At 60 drops per milliliter and 60 minutes per hour, the two sixties cancel, so the drip rate in drops per minute equals the rate in milliliters per hour.

Start from the volume, multiply by the drop factor written so that milliliters cancel, then divide by the time. Every unit except drops and minutes should cancel. If anything else is left over, a factor is upside down or missing.

The example below is one of DoseReady's own problems, worked by the app's code. The struck units are the ones that cancel.

Exams often give the time in hours. Add one more factor, 1 hr over 60 min, so hours cancel and minutes stay in the denominator. Skipping it leaves the answer sixty times too large, a slip the app names "Conversion skipped".

When the order is already a rate, such as a number of milliliters per hour, the chain starts from that rate. Multiply by the drop factor, then by 1 hr over 60 min. The drip rate practice problems include one worked this way.

Open RN's rounding guidance gives the reason: a drop cannot be given as a fraction of a drop, so drops are rounded to the nearest whole number. The WisTech dosage text also rounds drip rates to the nearest whole number, and Mercer County Community College's guidelines report manual drip rates in whole numbers.

Round once, at the end, under the rule your program states. Every example on this page uses "nearest whole number, half rounds up" for drops per minute, and says so next to the answer.

1. 1

   Conversion skipped: The time stays in hours instead of minutes, so the answer comes out sixty times too large. Check that the last unit standing is drops per minute.

2. 2

   Conversion factor inverted: The drop factor or the hours-to-minutes factor goes in upside down. The units stop canceling, which is the quickest way to spot it.

3. 3

   Decimal point one place out: The digits are right and the point is not. A drip rate ten times off is easy to catch if you ask whether you could count that many drops in a minute.

What is the formula for drops per minute?

Drops per minute equal the volume in milliliters times the drop factor in drops per milliliter, divided by the time in minutes. If the time is in hours, multiply the hours by 60 first.

Do you round drops per minute up or down?

Drops are counted whole, so drip rates are rounded to a whole number. Open RN, WisTech and Bergen round to the nearest whole number. Use the exact rule in your program's syllabus.

What is the difference between macrodrip and microdrip tubing?

Macrodrip tubing makes larger drops, commonly 10, 12, 15 or 20 drops per milliliter. Microdrip tubing makes 60 drops per milliliter, so its drip rate in drops per minute equals the rate in milliliters per hour.

How do you convert mL/hr to gtt/min?

Multiply the rate in milliliters per hour by the drop factor, then divide by 60 minutes per hour.

Where do you find the drop factor?

On the tubing package. On an exam it is given in the problem.

1. [1]Open RN, Nursing Skills 2e, 5.14 IV Infusion by Gravity (WisTech Open, 2023)wtcs.pressbooks.pub
2. [2]Open RN, Nursing Skills 2e, 5.13 Intravenous Infusions (WisTech Open, 2023)wtcs.pressbooks.pub
3. [3]Open RN, Nursing Skills 2e, 5.5 Rounding (WisTech Open, 2023)wtcs.pressbooks.pub
4. [4]WisTech Open, Nursing Dosage Calculations, Introduction to IV Flow Rates and Infusion Time (2024)wtcs.pressbooks.pub
5. [5]Bergen Community College Nursing, Pharmacological Math Computation Skills Self Learning Module (2023)bergen.edu
6. [6]Mercer County Community College Nursing, Medication Calculation Guidelines (updated September 2020)mccc.edu

Exam practice, never for patient care. Follow your program's rules.