
Common Drug Calculations

Medication administration is a core competency for nurses in every clinical setting. The ability to perform accurate dosage calculations is a key skill required to safely dispense drugs. New technologies such as bar coding medication and smart infusion pumps have helped to reduce medication errors (Cookson, 2013). However, nurses cannot rely completely on these advances. Two dosage calculation techniques are presented below: traditional formulas and dimensional analysis. Nurses should select one formula and practice to become proficient in that method.
Universal Formula (ToneyButler, 2023; Wilson, 2013)
All dosage calculations have these 2 components:
 Medication dosage prescribed by the healthcare provider
 Medication concentration supplied by the pharmacy
In the universal formula, the desired amount (D) is the dose prescribed by the provider. The amount on hand (H) is the dose on the container label. The volume (V) is the form and amount in which the drug is supplied (i.e. tablet, capsule, liquid). To use this formula, divide the desired amount by the amount on hand and multiply by the volume.
Example 1:
Administer digoxin 0.5 mg IV daily. The drug concentration available from the pharmacy is digoxin 0.25 mg/mL. How many mL will you need to administer a 0.5 mg dose?
D/H x V = Dose
0.5/0.25 x 1 = 2 mL
Intravenous (IV) Medications (Wilson, 2013)
Continuous IV drip calculations are more complex. Use the universal formula, recognizing that some conversions are usually necessary. First, determine the drug concentration. Then consider the unit in which your drug is measured (units/hour, mg/hour, mg/min, mcg/min, or mcg/kg/minute). Then, depending on how the drug is ordered, use one of the formulas below.
To find mcg/min:
If your amount on hand is in milligrams (mg), convert mg to micrograms (mcg) by multiplying by 1,000. Also, convert hours to minutes.
Example 2:
A patient is on a nitroglycerin drip. The IV pump is running at 8 mL/hour. The label on the bottle reads 50 mg in 500 mL 0.9% sodium chloride solution. What dose of nitroglycerin (mcg/min) is the patient receiving?
50 mg/500 mL x 1000 mcg/1 mg x 8 mL/1 hr x 1 hr/60 min
Answer: 13.3 mcg/min
Most institutions utilize infusion pumps that can be programmed to the tenth or hundredth decimal place. If your institution does not have infusion pumps with this capability, you may need to round to the nearest whole number.
To find mcg/kg/min:
Use the formula for mcg/min and divide by patient’s weight (kg).
Example 3: Administer dopamine at 10 mcg/kg/min. The pharmacy provides dopamine 800 mg in 250 mL of D5W. What is the hourly IV pump rate? The patient weighs 85.3 kg.
In this example, solve for mL/hr.
800 mg/250 mL x 1000 mcg/1mg x mL/hr x 1 hr/60 min ÷ 85.3 kg = 10 mcg/kg/min
800,000 mcg/250 mL x mL/hr x 1 hr/60 min ÷ 85.3 kg = 10 mcg/kg/min
3,200 mcg/mL x mL/hr x 1 hr/60 min ÷ 85.3 kg = 10 mcg/kg/min
3,200 mcg/mL x mL/hr x 1 hr/60 min = 853 mcg/min
mL/hr = 853 mcg/min x 1 mL/3,200 mcg x 60 min/hr
mL/hr = 16 mL/hr
To find units/hour:
First, determine the concentration of the amount on hand. Then, use the universal formula to calculate the rate.
Example 4:
Administer heparin 500 units per hour I.V. The pharmacy supplies the heparin infusion as 20,000 units in 500 mL D_{5}W.
Find the concentration:
20,000 units/500 mL = 40 units/mL
Use the universal formula:
D/H x V = Dose
500 units/hr ÷ 40 units/mL = 12.5 mL/hour