Accepted Practice

Calculating intravenous flow rates

Delivery of the correct medication, dose, and volume at the appropriate infusion rate and time is essential for safe and therapeutic intravenous (IV) medication administration. Today’s IV infusion pumps can make this process seem simple. They deliver precise volume-controlled infusions, and many can be programmed to calculate dose and flow rates. Despite these conveniences, knowing how to calculate IV flow rates correctly will help you verify equipment accuracy and help prevent adverse events related to medication errors. Knowing how to perform these simple calculations is also helpful when a programmable pump is unavailable, not to mention when calculations are part of pre-employment testing.

The first step in determining IV flow-rate calculations is to check the medication label. Compare the label to the medication administration record (MAR) for the correct patient, medication, dose, time, and route. You should perform this comparison a total of three times before you begin the infusion.

IV medications are diluted in a variety of concentrations and delivered in a variety of dose rates. Be sure to clarify any questionable orders and use only approved abbreviations to avoid dangerous adverse events.

Appropriate IV-medication infusion orders specify the dose to be given over a specific interval and the concentration of the drug in solution. You must calculate the unknown flow rate. There are three factors involved in performing calculations for IV medication infusions. If you know two factors, you can calculate the third by using the basic formula:

The concentration of medications is the amount of drug diluted in a given volume of IV solution, usually measured in units, micrograms (mcg), milligrams (mg), or grams (gm).

The dose of the medication is the amount of drug ordered for infusion over a specific length of time. Doses have varying units of measurement. The length of time is either by the minute or by the hour. If the medication is dosed by weight, the calculation is made using the patient’s weight in kilograms (kg).

The flow rate determines how rapidly the infusion is delivered to the patient. On an infusion pump, the flow rate is set using using mL/hr. But you will not always have an infusion pump available, in which case you will have to adjust the rate manually in drops per minute. A basic formula for calculating an IV flow rate in drops per minute without medications is:

For example, the provider has ordered 1,000 mL of 0.9% sodium chloride (normal saline) to infuse over 8 hr. You have macrodrip tubing with a drop factor of 15 gtt/mL. You must calculate how many gtt/min to use to set the IV flow rate.

Enter the known factors into the formula and solve.

Solving the equation, you first have:

Then reduce the fraction, and multiply. The IV flow rate is 31.2, or 31 gtt/min.

When you are administering IV medications and must calculate rates, you need the following data:

  • The unit of measurement used for the drug (units, mg, mcg)
  • The dose to be delivered by unit of measurement (gtts, units, mg, mcg, dose/kg)
  • The volume of the diluent (mL)
  • The time over which each unit of drug is to be delivered (minutes, hours)
  • The patient’s weight in kilograms (required for some medications)

To calculate an unknown flow rate, use this formula:

Step 1: Convert the drug concentration to a like unit of dose measurement.
Step 2: Convert the desired flow rate to an hourly rate if necessary.
Step 3: Calculate the concentration of the drug in 1 mL of fluid.
Step 4: Enter the known and calculated factors into the formula and solve.

Example #1 mg/hr

In this example, the units of measurement are already alike. You must determine only the drug concentration per mL, enter the factors into the formula, and solve.

Example #2: units/hr

Once again, the units of measurement are alike, only in units rather than mg. You must determine the drug concentration per mL.

Example #3: mg/min

In this example, you must convert the dose time from minutes to hours and determine the drug concentration per mL.

Example #4: mcg/min

For this calculation, you must convert the drug concentration to a like unit of measurement, determine the drug concentration per mL, and convert the rate per minute to the rate per hour.

First convert the concentration to like units of measurement (mg to mcg) and then determine the drug concentration per mL.

Enter the known factors into the formula, convert the time to hourly, and solve.

Example #5: mcg/kg/min

The basic calculation is the same; however, the weight is factored in:

First convert the drug concentration to the like unit of the dose (mg to mcg) and then determine the drug concentration per mL.

Enter the known factors into the formula and solve.

Example #6: gtt/min

When delivering fluid without an infusion pump, you must check the IV tubing manufacturer’s specifications to determine the drop factor (how many drops per minute the infusion set delivers).

Prior to starting the infusion, always double-check your calculations. Ask another qualified person to check your results with you if your agency policy requires it or if you are unsure of your results.

Finally, remember that errors can and do happen. When assuming care for a patient with an active IV infusion, compare the infusing IV solutions with the patient’s medication record. By performing a few simple calculations, you can check the accuracy of the infusion device, prevent medication errors, and ensure optimal patient safety during IV medication therapy.