# PHARMACIST ADVISOR: Calculating drips per minute

Q: I am a traveling nurse who works in critical care, where IV drips are an everyday thing. Most of the hospitals I work in have monitors, but calculating the number of drops per minute (gtt/min) from the doctor's order written in ml/hr is a goal I need to master. Could you please send me a simple formula for calculating drips? Thanks.

A: There are three things you need to know before you can calculate from ml/hr to drops/min (also written as "gtt/min"):

1. The original order, written as "ml/hr" or "ml/min"
2. The type of fluid or medication to be infused – some medications must be infused slowly, whereas fluids (such as D5W) may be infused more rapidly
3. The type of tubing you will need to use – there are 3 sizes of tubing: 10 gtt/min, 15 gtt/min, and 60 gtt/min ("Microdrip"). The type of tubing you choose will depend on the medication or fluid you are infusing (i.e., a liter of normal saline solution (NSS), an antibiotic, or total parenteral nutrition [TPN]), as well as the type of patient (i.e., adult, pediatric)

When you set up your patient's IV drip, you will be counting the number of drops that fall over 1 minute, so our final answer must be in drops per minute (gtt/min). To calculate gtt/min, follow these steps:

Step 1: Convert the rate of administration from ml/hr to ml/min
1 hour = 60 minutes, so take your number in ml/hr and divide by 60 min/hr.

For example: You have an order for D5W 1000 ml over 8 hours.
First, 1000 ml / 8 hr = 125 ml/hr; 125 ml/hr / 60 min = 2.08 ml/min

Math tip: use conversions in which the units cancel to find your answer:
(1000 ml/8 hr) x (1 hr/60 min) = 1000 ml/480 min = 2.08 ml/min
(Note how "hr" gets cancelled from the denominator and numerator)

Step 2: Choose the tubing
Remember, just as with needle gauge size, the smaller the number, the bigger the tubing.

• For any rate greater than 120ml/hr use the regular tubing which is available as 10gtt/ml or 15gtt/ml. These kinds of tubing are used for infusing:
– large volume parenteral infusions or TPN
– IV fluids (i.e., NSS, D5W)
– Antibiotics (i.e., Cipro, Levaquin)
• Microdrip tubing (60 gtt/ml) delivers very small drops may be a good choice when you need a very slow infusion, or you are using a very small volume (i.e. pediatric patients).

Step 3: Calculate drops/min based on the tubing

Multiply the rate of administration in ml/min by the type of tubing you're using:
gtt/min = (ml/min) x (gtt/ml)

For example: In Step 1, you found that the rate of administration for D5W 1000 ml over 8 hours
was 2.08 ml/min. You want to use tubing that delivers 15 gtt/ml:

gtt/min = (2.08 ml/min) x (15 gtt/ml)
gtt/min = 31 gtt/min

(Note how "ml" gets cancelled from the numerator and denominator)

FORMULA

To do all of these calculations in one step:

gtt/min = original order (ml/hr) x conversion hrs to mins (1 hr/60 min) x tubing (gtt/ml)

More simply:
gtt/min = (ml/hr) x (1 hr/60 min) x (gtt/ml)

(Note how "hr" and "ml" cancel out in the numerator and denominator)

Practice Problem #1: MD orders NS to be run at 150 ml/hr continuous, using 15 gtt/ml tubing. You don't have an infusion monitor. Find the correct rate of administration in drops/minute.

Answer: (150 ml/hr) x (1 hr/60 min) x (15 gtt/ml) = 37.5 ml ~ 38 gtt/min

Practice Problem #2: MD orders Nafcillin 1 g in 30 ml NSS to be administered over 1 hour. Find the correct rate of administration in drops/minute using 15 gtt/ml tubing and 60 gtt/ml tubing.

Answer (15 gtt/ml tubing): (30 ml/1 hr) x (1 hr/60 min) x (15 gtt/ml) = 7.5 gtt/min

Answer (60 gtt/ml tubing): (30 ml/1 hr) x (1 hr/60 min) x (60 gtt/ml) = 30 gtt/min

As you can see, using the Microdrip tubing for a small volume can make it easier to count the drops per minute. Most importantly, your measurements are likely to be more accurate with the Microdrip in this case.

Use this handy chart to convert from ml/hr to gtt/min with the 3 different kinds of tubing:

Send your questions about medications to Marcy and Larry Gever at marcygever@home.com. They'll respond to the most notable questions in future Pharmacist Advisor columns.