Nursing Dosage Calculations You Need to Know for the NCLEX-RN

Nursing dosage calculations are a critical part of medication administration in healthcare. They are also an essential component of passing the NCLEX exam.

This article will explain what you should know about nursing dosage calculations for the NCLEX, including units of measurement, basic dosing calculations, pediatric dosage calculations, IV flow rates, and other tips for success!

Why You Need to Know Nursing Dosage Calculations for the NCLEX

The NCLEX is a standardized test that nursing students must pass to become licensed nurses. The exam will assess your ability to provide safe and effective nursing care, including medication administration. Therefore, nursing dosage calculations are a critical aspect of the NCLEX exam.

On the NCLEX, you may encounter questions that require you to calculate medication dosages. These questions include the correct amount of medication to administer, the rate of medication infusion, and the duration of medication administration. They test your ability to perform mathematical calculations accurately, as well as your knowledge of medication dosages, routes of administration, and the patient's weight and age.

Nursing Units of Measurement

The three main units of measurement in nursing include the apothecary system, the household system, and the metric system.

1. The Apothecary System

The apothecary system is an older system of measurement that has largely been replaced by the metric system. It includes units such as grains (gr), drops (gtt), and drams (dr).

Here are some common units of measurement used in the apothecary system and their approximate equivalents in metric units:

• Grain (gr): approximately 65 milligrams (mg)

• Dram (dr): approximately 4 grams (g) or 60 grains

• Ounce (oz): approximately 28 grams (g) or 8 drams

• Pound (lb): approximately 454 grams (g) or 16 ounces

• Fluid Dram (fl dr): approximately 3.7 milliliters (mL)

• Fluid Ounce (fl oz): approximately 29.6 milliliters (mL)

• Pint (pt): approximately 473 milliliters (mL) or 16 fluid ounces

• Quart (qt): approximately 946 milliliters (mL) or 2 pints

• Gallon (gal): approximately 3.8 liters (L) or 4 quarts

2. The Household System

The household system is based on common household measurements and includes units such as teaspoons (tsp), tablespoons (tbsp), and ounces (oz). 

Here are some common household units of measurement and their approximate equivalents:

• Teaspoon (tsp): approximately 5 milliliters (mL)

• Tablespoon (tbsp): approximately 15 milliliters (mL) or 3 teaspoons

• Fluid ounce (fl oz): approximately 30 milliliters (mL) or 2 tablespoons

• Cup: approximately 240 milliliters (mL) or 8 fluid ounces

• Pint: approximately 480 milliliters (mL) or 2 cups

• Quart: approximately 960 milliliters (mL) or 4 cups or 2 pints

• Gallon: approximately 3.8 liters (L) or 16 cups or 8 pints or 4 quarts

The apothecary and household systems of measurement are rarely used for nursing dosage calculations in hospital settings, though they may be used in home healthcare settings. Instead, this article will focus mainly on the metric system units of measurement.

3. The Metric System

The metric system is the standard unit of measurement used in US healthcare and is also known as The International System of Units (SI). It is a decimal-based system that is more precise compared to the apothecary and household systems, both of which may lead to errors in medication dosage if used in healthcare settings.

The three types of metric system units that are commonly used to measure medications are as follows: 

• Volume: liter (L), milliliter (mL)

• Mass/weight: gram (g), kilogram (kg), milligram (mg)

• Length/distance: meter (m), centimeter (cm), millimeter (mm)

Volume: Liter (L) And Milliliter (mL)

A liter (L) is a metric unit of volume that is commonly used for measuring liquids. 

• Liter (L): The base metric unit of volume, equal to 1,000 milliliters

• Milliliter (mL): 1/1000 of a liter

Examples of liter dosages that nurses may encounter in their practice include:

• Intravenous (IV) fluids: A common order for IV fluids is to administer 1 liter over a specific time period, such as 8 hours. The rate of administration would then be calculated based on the prescribed time frame.

Milliliter dosages are 1000 times smaller than liter dosages. Examples of milliliter dosages nurses may encounter in their practice may include:

• A medication comes in a liquid form to be taken orally with a concentration of 10 mg/mL. The prescribed dose is 50 mg of the medication, which means that the nurse will need to measure out 5 mL of the medication using a syringe or other measuring device. This is because 50 mg ÷ 10 mg/mL = 5 mL.

Mass/Weight: Gram (g), Kilogram (kg), Milligram (mg)

Healthcare professionals use grams to measure the mass of small amounts of medications. The metric system most commonly uses kilograms (the largest), grams, and milligrams (the smallest) to measure mass.

• Milligram (mg): 1/1000 of a gram

• Gram (g): the base unit of mass in the metric system

• Kilogram (kg): 1,000 grams or 2.2 pounds

Examples of gram dosages that nurses may encounter in their practice include:

• Antibiotic medications: Many antibiotics are prescribed in gram dosages. For example, a common dose of amoxicillin for an adult may be 1 gram, taken orally every 12 hours.

• Medications for Hyperkalemia: Medications such as Kayexalate are prescribed to help lower potassium levels. A typical dose may be 15-30 grams orally, four times per day.

Length/Distance: Meter (m), Centimeter (cm), Millimeter (mm)

The meter is used to measure length, such as a person’s height. 

The meter is typically not a unit of measurement for medication or medical treatment. However, nurses may use the meter to measure the length of wounds, the distance a patient can walk, or the size of medical equipment such as catheters or tubes.

Importance of Understanding Units of Measurement for the NCLEX

The NCLEX is required to earn licensure to practice in the US, and understanding units of measurement are crucial for passing it. Nurses must be able to safely measure and administer medications, fluids, and other substances to patients. Even minor errors in measurement can have serious consequences.

The NCLEX includes questions that test a nurse's ability to convert between different units of measurement, calculate medication dosages, and interpret laboratory results, so it is essential for nursing students to have a solid understanding of this topic.

How to Convert Between Units of Measurement

Converting Between Milliliters (mL) and Liters (L)

To convert from milliliters to liters, you need to divide the number of milliliters by 1000. For example, if a patient is receiving a 500 mL bag of intravenous (IV) fluids, you can convert that to liters by dividing 500 by 1000 to get 0.5 L.

• mL to L equation: mL / 1000 = L

• Example: 500 mL / 1000 = 0.5 L

To convert from liters to milliliters, you need to multiply the number of liters by 1000. For example, if a patient is receiving 2 L of oxygen per minute via nasal cannula, you can convert that to milliliters per minute by multiplying 2 by 1000 to get 2000 mL per minute.

• L to mL equation: L x 1000 = mL

• Example: 2 L x 1000 = 2000 mL

Converting Between Milligrams (mg) and Grams (g)

To convert from milligrams to grams, you need to divide the number of milligrams by 1000. For example, if a medication is prescribed at a dose of 500 mg, you can convert that to grams by dividing 500 by 1000 to get 0.5 g.

• Mg to g equation: mg / 1000 = g

• Example:  500 mg / 1000 = 0.5 g

To convert from grams to milligrams, you need to multiply the number of grams by 1000. For example, if a patient is prescribed medication at a dose of 2 g, you can convert that to milligrams by multiplying 2 by 1000 to get 2000 mg.

• G to mg equation: g x 1000 = mg

• Example: 2 g x 1000 = 2000 mg

Converting Between Pounds (lb) and Kilograms (kg)

To convert from pounds to kilograms, you need to divide the number of pounds by 2.2. For example, if a patient weighs 154 lb, you can convert that to kilograms by dividing 154 by 2.2 to get 70 kg.

• Lb to kg equation: lb / 2.2 = kg

• Example: 154 lb / 2.2 = 70 kg

To convert from kilograms to pounds, you need to multiply the number of kilograms by 2.2. For example, if a patient weighs 50 kg, you can convert that to pounds by multiplying 50 by 2.2 to get 110 lb.

• Kg to lb equation: kg x 2.2 = lb

• Example: 50 kg x 2.2 = 110 lb

Basic Dosage Calculations

It is important to be able to calculate medication dosages accurately and safely. One of the most basic medication formulas used for nursing dosage calculations is:

Dose ordered x volume to be administered = Amount to administer

Here is what each part of the formula means:

Dose ordered: This is the amount of medication that the healthcare provider has prescribed for the patient. It's usually expressed in milligrams (mg) or grams (g) for solid medications, or milliliters (mL) for liquid medications. 

For example, if the healthcare provider orders a dose of 500 mg of a medication, this would be the "dose ordered" in the formula.

Volume to be administered: This is the amount of medication that will be given to the patient in a single dose. It's usually expressed in milliliters (mL) for liquid medications, but can also be expressed in other units of measurement depending on the medication. 

For example, if the medication is available in a 10 mL vial, and the healthcare provider orders a dose of 5 mL, this would be the "volume to be administered" in the formula.

Amount to administer: This represents the actual amount of medication that will be given to the patient in a single dose. It's also usually expressed in the same units of measurement as the "dose ordered" (mg or g for solid medications, mL for liquid medications). 

To calculate the "amount to administer," you simply multiply the "dose ordered" by the "volume to be administered" using the formula.

The Importance of Checking Calculations

The NCLEX tests a nurse's ability to provide safe and effective care to patients. This includes the ability to accurately calculate medication dosages and administer medications safely. Therefore, double-checking calculations is not only important for patient safety but also for passing the NCLEX and becoming a licensed nurse.

When studying nursing dosage calculations for the NCLEX, it is wise to get into the habit of re-checking your work to ensure it is correct. This habit will help you perform better on the NCLEX and provide the right response.

Pediatric Dosage Calculations

Pediatric dosing calculations require more careful attention to detail and precision, as the dosing needs of children can vary significantly based on their weight, age, and developmental stage.

Body Weight

Children's weight varies widely, and pediatric dosages are typically calculated based on the child's weight. Comparatively, adult dosages are often calculated based on standardized dosing schedules.

Prescribing medications based on the child’s weight helps ensure that the dosage is appropriate and safe for the child's size and age.

Physiological Differences 

Children's organs are still developing, so they may have different metabolic rates compared to adults. This means that medications may be absorbed, distributed, metabolized, and eliminated differently in children than adults.

Age

Infants and young children have different physiological characteristics than older children and adults. Therefore, children of varying age groups often have specific dosing requirements. 

Medication Form

Many pediatric drugs are formulated as liquids rather than pills or tablets. Liquid medication requires different calculations, as you must measure its volume in milliliters rather than in milligrams or grams.

Examples of How to Calculate Pediatric Dosages for the NCLEX

Here are a few examples of how to calculate pediatric dosages for the NCLEX:

Question 1:

A child weighing 33 lbs needs a medication that is ordered at a dose of 25 mg/kg/day in divided doses every 6 hours. The medication is available in a concentration of 50 mg/5 mL. How many milliliters should the child receive per dose?

Step 1: Convert the child's weight from pounds to kilograms

33 lbs ÷ 2.2 = 15 kg

Step 2: Calculate the total daily dose

25 mg/kg/day x 15 kg = 375 mg/day

Step 3: Calculate the amount per dose

375 mg/day ÷ 4 doses per day = 93.75 mg/dose

Step 4: Calculate the volume of medication to administer

93.75 mg/dose ÷ 50 mg/5 mL = 9.375 mL/dose

Round to the nearest tenth: 9.4 mL/dose

Therefore, the child should receive 9.4 mL of medication per dose.

Example 2:

A child weighing 22 lbs needs a medication that is ordered at a dose of 12 mg/kg/day in divided doses every 8 hours. The medication is available in a concentration of 40 mg/mL. How many milliliters should the child receive per dose?

Step 1: Convert the child's weight from pounds to kilograms

22 lbs ÷ 2.2 = 10 kg

Step 2: Calculate the total daily dose

12 mg/kg/day x 10 kg = 120 mg/day

Step 3: Calculate the amount per dose

120 mg/day ÷ 3 doses per day = 40 mg/dose

Step 4: Calculate the volume of medication to administer

40 mg/dose ÷ 40 mg/mL = 1 mL/dose

Therefore, the child should receive 1 mL of medication per dose.

IV Flow Rate Calculations

Calculating intravenous (IV) flow rates is essential to ensure the patient gets the correct amount of medication or fluid therapy. IV flow rate calculation questions will also be on the NCLEX. The formula for calculating IV flow rate is:

Flow rate (mL/hr) = volume to be infused (mL) / time (hr)

For example, if the total volume to be infused is 1000 mL over 8 hours, the IV flow rate in mL per hour would be:

• IV flow rate (mL/hr) = 1000 mL / 8 hours = 125 mL/hr

And the IV flow rate in mL per minute would be:

• IV flow rate (mL/min) = 125 mL/hr / 60 = 2.08 mL/min (rounded to two decimal places)

Sample Question: A patient needs 500 mL of saline solution infused over a period of 8 hours. What is the IV flow rate in mL/hr?

Answer: Using the formula Flow rate (mL/hr) = volume to be infused (mL) / time (hr), we can calculate the IV flow rate as follows:

Flow rate (mL/hr) = 500 mL / 8 hr

Flow rate (mL/hr) = 62.5 mL/hr

Therefore, the IV flow rate for this patient would be 62.5 mL/hr.

Special Considerations

When calculating medication dosages for elderly patients or patients with renal or hepatic impairment, you should keep some special considerations in mind for the NCLEX:

Age-related changes: As patients age, their metabolism and organ function may decline. Elderly patients may require lower medication dosages.

Renal impairment: Patients with renal impairment may need medication dosages adjusted or avoided if the medication is primarily eliminated by the kidneys.

Hepatic impairment: Patients with hepatic impairment may need medication dosages adjusted or avoided if the medication is metabolized mainly by the liver.

Drug interactions: It is important to consider potential drug interactions and carefully review the patient's medication list to avoid adverse drug reactions.

How to Calculate Dosages With Special Considerations

Nurses must carefully assess each patient's needs and medical history when calculating medication dosages. This assessment may involve consulting with a pharmacist, using specialized formulas or calculators, and monitoring the patient's response to medication closely.

If you are ever concerned or have a question about a nursing dosage, contacting the prescribing healthcare provider or the hospital pharmacist is vital. It is always better to communicate and be safe than sorry when giving patients medication. 

Changing medication dosages without a written order is outside a nurse's scope of practice. It may result in patient harm, disciplinary action from their state board of nursing, and legal liability.

3 Tips for Dosage Calculation Success on the NCLEX

Practicing dosage calculations is crucial for success on the NCLEX because medication administration is a fundamental aspect of nursing practice. In addition, the NCLEX is known to have a significant number of dosage calculation questions, so practicing these types of questions can improve your chances of passing the exam.

1. Use NCLEX Study Resources

There are many online resources are available if you want nursing dosage calculation practice in preparation for the NCLEX. Some resources include:

• Lippincott

• RegisteredNurse.com

• Nurse Labs

• LevelUp RN

• Nurse Plus Academy

2. Practice, Practice, Practice!

The best way to prepare for nursing dosage calculations on the NCLEX is to practice as many questions as possible. Use NCLEX practice questions, review books and online resources, or join a study group.

3. Understand the Test Format

You should familiarize yourself with the NCLEX format and the types of questions it asks. The NCLEX is a computer-adaptive test, meaning the questions will get harder or easier depending on how well you're doing.

Nursing Dosage Calculations For NCLEX Success

Nursing dosage calculations may seem intimidating, but you've got this! With dosage calculation practice and preparation, you'll easily tackle these equations and pass the NCLEX. 

You have worked hard to get to this point in your nursing program, and you are capable of mastering dosage calculations and conquering the NCLEX exam. Best of luck!

Interested in more NCLEX study tips? Check out nurse.org’s comprehensive NCLEX resources to learn everything you need to know about the NCLEX exam.