Nurse Practitioner - Owner

Doctor of Naturopathy

Lehigh Valley Wellness Center

Many nurses are weak with drug calculations of all sorts. This article will help to review the major concepts related to drug calculations, help walk you through a few exercises, and provide a few exercises you can perform on your own to check your skills. There are many reference books available to review basic math skills, if you find that you have difficulty with even the basic conversion exercises.

**Common Conversions:**

1 Liter = 1000 Milliliters

1 Gram = 1000 Milligrams

1 Milligram = 1000 Micrograms

1 Kilogram = 2.2 pounds

**Methods of Calculation**

Any of the following three methods can be used to perform drug calculations. Please review all three methods and select the one that works for you. It is important to practice the method that you prefer to become proficient in calculating drug dosages.

**Remember:** Before
doing the calculation, convert units of measurement to one system.

**I. Basic Formula:**Frequently
used to calculate drug dosages.

**D = dose ordered or desired dose
H = dose on container label or dose on hand
V = form and amount in which drug comes (tablet, capsule, liquid)**

**Example:** Order-Dilantin
50mg p.o. TID Drug available-Dilantin 125 mg/5ml

**II. Ratio & Proportion:** Oldest
method used in calculating dosage.

Left side are known quantities. Right side is desired dose and amount to give

Multiply the means and the extremes

**Example:** Order-Keflex
1 gm p.o. BID Drug available-Keflex 250 mg per capsule

**III. Fractional Equation**

H

V = D

X

Cross multiply and solve for X.

** Example:** Order - Digoxin
0.25 mg p.o. QD Drug Available - 0.125 mg per tablet

**IV. Intravenous Flow Rate Calculation (two methods)**

** Two Step**

Step 1 - Amount of fluid divided by hours to administer = ml/hr

** One Step**

**Example:** 1000 ml over
8 hrs IV set = 15 gtts/ml

Two Step

Step 1 - 1000 divided by8 = 125

Step 2 -

One Step

**IV. How to Calculate Continuous Infusions**

**B. mcg/min** (For example
- Nitroglycerin)

**C. mcg/kg/min** (For
example - Dopamine, Dobutamine, Nipride, etc.)

1.) To calculate **cc/hr** (gtts/min)

**Example:** Dopamine 400
mg/250 cc D5W to start at 5 mcg/kg/min. Patient’s weight is 190 lbs.

2. To calculate **mcg/kg/min**

**Example:** Nipride 100mg/250
cc D 5 W was ordered to decrease your patient’s blood pressure. The patient's weight is 143 lbs, and the IV pump is set at
25 cc/hr. How many mcg/kg/min of Nipride is the patient receiving?

**V. How to calculate mcg/kg/min if you know the
rate of the infusion**

**For example:**

400mg of Dopamine in 250 cc D 5 W = 1600 mcg/cc÷ 60 min/hr= 26.6 mcg/cc/min

26.6 is the dosage concentration for Dopamine in mcg/cc/min based on having 400mg in 250cc of IV fluid. You need this to calculate this dosage concentration first for all drug calculations. Once you do this step, you can do anything!

**NOW DO THE REST!**

(If you have a 75 kg patient for example...)

**=
3.5 mcg/kg/min (rounded down)**

**B. How to calculate drips in cc per hour when
you know the mcg/kg/min that is ordered or desired**

**For example:**

400 mg Dopamine in 250 cc DW = 26.6 mcg/cc/min

** ALWAYS WORK THE EQUATION BACKWARDS AGAIN TO DOUBLE
CHECK YOUR MATH! **

**For example:**

400mg of Dopamine in 250 cc DW = 1600 mcg/cc 60 min/hr = 26.6 mcg/cc/min

26.6 is the dosage concentration for Dopamine in mcg/cc/min based on having 400 mg in 250 cc of IV fluid. You need this to calculate this dosage concentration first for all drug calculations. Once you do this step, you can do anything!

**NOW DO THE REST!!**

(If you have a 75 kg patient for example)

**Now do some practice exercises to check what you
learned**

**A. Practice Problems:**

- 1. 2.5 liters to milliliters
- 2. 7.5 grams to milligrams
- 3. 10 milligrams to micrograms
- 4. 500 milligrams to grams
- 5. 7500 micrograms to milligrams
- 6. 2800 milliliters to liters
- 7. 165 pounds to kilograms
- 8. 80 kilograms to pounds

**B. Practice Problems:** Use
the method you have chosen to calculate the amount to give.

- 1. Order-Dexamethasone 1 mg Drug available-Dexamethasone 0.5 mg per tablet
- 2. Order-Tagamet 0.6 gm Drug available-Tagamet 300 mg per tablet
- 3. Order-Phenobarbital 60 mg Drug available-Phenobarbital 15 mg per tablet
- 4. Order-Ampicillin 0.5 gm Drug available-Ampicillin 250 mg per 5 ml
- 5. Order-Dicloxacillin 125 mg Drug Available-Dicloxacillin 62.5 mg per 5 ml
- 6. Order-Medrol 75 mg IM Drug Available-Medrol 125 mg per 2 ml
- 7. Order-Lidocaine 1 mg per kg Patient's weight is 152 pounds
- 8. Order- 520 mg of a medication in a 24 hour period. The drug is ordered every 6 hours. How many milligrams will be given for each dose?

**C. Practice Problems:**

- 1. Order-1000 ml over 6 hrsIV set 15 gtts/ml
- 2. Order-500 ml over 4 hrs IV set 10 gtts/ml
- 3. Order-100 ml over 20 min. IV set 15 gtts/ml

**D. Practice Problems:**

- 1. Dopamine 400 mg in 250 cc DW to infuse at 5 mcg/kg/min. The patient’s weight is 200 pounds. How many cc/hour would this be on an infusion pump?
- 2. A Dopamine drip (400mg in 250 cc of IV fluid) is infusing on your 80 kg patient at 20 cc/hour. How many mcg/kg/min are infusing for this patient?
- 3. A Nitroglycerin drip is ordered for your patient to control his chest pain. The concentration is 100 mg in 250 cc D 5 W. The order is to begin the infusion at 20 mcg/min. What is the rate you would begin the infusion on the infusion pump?
- 4. A Nitroglycerin drip (100mg in 250 cc DW) is infusing on your patient at 28 cc/hour on the infusion pump. How many mcg/min is your patient receiving?
- 5. A procainamide drip is ordered (2gms in 250 cc D5W) to infuse at 4 mg/min. The patient weighs 165 pounds. Calculate the drip rate in cc/hour for which the infusion pump will be set at.
- 6. A Lidocaine drip is infusion on your 90 kg patient at 22 cc/hour. The Lidocaine concentration is 2 grams in 250 cc of D 5 W. How many mg/min is your patient receiving?

**Summary**

Many nurses have difficulty with drug calculations. Mostly because they don’t enjoy or understand math. Practicing drug calculations will help nurses develop stronger and more confident math skills. Many drugs require some type of calculation prior to administration. The drug calculations range in complexity from requiring a simple conversion calculation to a more complex calculation for drugs administered by mcg/kg/min. Regardless of the drug to be administered, careful and accurate calculations are important to help prevent medication errors. Many nurses become overwhelmed when performing the drug calculations, when they require multiple steps or involve life-threatening drugs. The main principle is to remain focused on what you are doing and try to not let outside distractions cause you to make a error in calculations. It is always a good idea to have another nurse double check your calculations. Sometimes nurses have difficulty calculating dosages on drugs that are potentially life threatening. This is often because they become focused on the actual drug and the possible consequences of an error in calculation. The best way to prevent this is to remember that the drug calculations are performed the same way regardless of what the drug is. For example, whether the infusion is a big bag of vitamins or a life threatening vasoactive cardiac drug, the calculation is done exactly the same way.

Many facilities use monitors to calculate the infusion rates, by plugging the numbers in the computer or monitor with a keypad and getting the exact infusion titration chart specifically for that patient. If you use this method for beginning your infusions and titrating the infusion rates, be very careful that you have entered the correct data to obtain the chart. Many errors take place because erroneous data is first entered and not identified. The nurses then titrate the drugs or administer the drugs based on an incorrect chart. A method to help prevent errors with this type of system is to have another nurse double check the data and the chart, or to do a hand calculation for comparison. The use of computers for drug calculations also causes nurses to get “rusty” in their abilities to perform drug calculations. It is suggested that the nurse perform the hand calculations from time to time, to maintain her/his math skills.

- 1. 2500 mL
- 2. 7500 mg
- 3. 10,000 mcg
- 4. 0.5 gm
- 5. 7.5 mg
- 6. 2.8 L
- 7. 75 kg
- 8. 176 lbs

- 1. 41.6 (42)
- 2. 20.8 (21)
- 3. 75

- 1. 2 tablets
- 2. 2 tablets
- 3. 4 tablets
- 4. 10 mL
- 5. 10 mL
- 6. 1.2 mL
- 7. 69 kg = 69 mg
- 8. 130 mg for 4 doses

- 1. 17 cc/hr
- 2. 6.65 mcg/kg/min
- 3. 3 cc
- 4. 186.5 mcg/min
- 5. 30 cc/hr
- 6. 3 mg/min

**Reference:** Dosage Calculations Made Incredibly Easy! by Springhouse Corporation, 1998

Louise Diehl, RN, MSN, ND, ACNS-BC, NP-C

Nurse Practitioner - Owner

Doctor of Naturopathy

Phone: 908-213-8808

Fax: 908-213-8898

Lehigh Valley Wellness Center

101 Coventry Drive

Phillipsburg, NJ 08865

E-mail: __info@lehighvalleywellnesscenter.com__

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