Brush up on Your Drug Calculation Skills
Louise Diehl, RN, MSN, ND, ACNSBC, NPC
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: OrderDilantin 50mg p.o. TID
Drug availableDilantin 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: OrderKeflex 1 gm p.o. BID
Drug availableKeflex 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 by 8 = 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.)

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 D_{5}W = 26.6 mcg/cc/min
ALWAYS WORK THE EQUATION BACKWARDS AGAIN TO DOUBLE CHECK YOUR MATH!
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 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. OrderDexamethasone 1 mg
Drug availableDexamethasone 0.5 mg per tablet
2. OrderTagamet 0.6 gm
Drug availableTagamet 300 mg per tablet
3. OrderPhenobarbital 60 mg
Drug availablePhenobarbital 15 mg per tablet
4. OrderAmpicillin 0.5 gm
Drug availableAmpicillin 250 mg per 5 ml
5. OrderDicloxacillin 125 mg
Drug AvailableDicloxacillin 62.5 mg per 5 ml
6. OrderMedrol 75 mg IM
Drug AvailableMedrol 125 mg per 2 ml
7. OrderLidocaine 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. Order1000 ml over 6 hrs IV set 15 gtts/ml
2. Order500 ml over 4 hrs IV set 10 gtts/ml
3. Order100 ml over 20 min. IV set 15 gtts/ml
D. Practice Problems:
1. Dopamine 400 mg in 250 cc D_{5}W 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 D_{5}W) 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 lifethreatening 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.
Answers to Practice Problems
A. Practice Problems
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
B. Practice Problems
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
C. Practice Problems
1. 41.6 (42)
2. 20.8 (21)
3. 75
D. Practice Problems
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
