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Drug Calculations

Unit 6 Lesson 3

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Explain and demonstrate calculation methods
Provide an example of how each calculation method is used
Explain when and how to round medication doses
Provide examples of rounding doses for each of the above formula method

Student Learning Outcomes

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Calculation Methods

Basic Formula

Simple one-step calculation
Uses desired and available doses
Good for tablet calculations

Ratio/Proportion

Sets up equivalent relationships
Uses cross-multiplication
Helps with complex conversions

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Calculation Methods continued…

Dimensional Analysis

Converts between multiple units
Eliminates confusion with units
Excellent for IV calculations

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When to Use Each Method

Basic Formula

Simple doses
Tablets
Liquids

Ratio & Proportion

Multiple concentrations/volumes

Dimensional Analysis

Complex conversions
IV rates

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When to Use Each Method continued…

Body Weight

Pediatric
Weight-based medications

Body Surface Area

Chemotherapy
Pediatric dosing

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Basic Formula Method (Desire over Have)

Formula� (Desired / Have) × Quantity = Amount

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Basic Formula Method (Desire over Have)

Identify desired dose (ordered)

Identify dose on hand (available)
Divide desired by have
Multiply by quantity/volume
Apply rounding rules

Safety Note: Always check units match before calculating

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Basic Formula Example

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Practice Problems

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Ratio and Proportion Method

Formula

Known dose :: Known unit = Desired dose :: X

OR

Known dose / Known unit = Desired dose / X

Set up equivalent ratios
Ensure units match
Cross multiply
Solve for X
Apply rounding rules

Safety Note: Double-check your units before solving

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Ratio and Proportion Example

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Practice Problems

Solve for the dosage (X)

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Dimensional Analysis Method

Formula

Starting quantity × Conversion factor 1 × Conversion factor 2 = Desired answer

Identify what you need to find
Start with ordered amount
Set up conversion factors
Cancel out unwanted units
Solve and round appropriately

Safety Note: This method reduces errors in complex calculations

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Dimensional Analysis Example 1

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Dimensional Analysis Example 2

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Practice Problems

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Body Weight Method

Formula

Dose = Weight (kg) × Dose per kg

Convert weight to kg
Multiply by ordered dose
Calculate final amount
Apply rounding rules

Safety Note: Weight-based dosing reduces medication errors in clients of different sizes

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Body Weight Method Example

Clinical Scenario

A 6-year-old with strep throat (weight 110 lb) needs amoxicillin 10 mg/kg QID.

Available: Amoxicillin 125 mg/5 mL.

Solution

Convert: 110 lb / 2.2 = 50 kg
Calculate dose: 10 mg/kg × 50 kg = 500 mg
Calculate volume: 500 mg × (5 mL / 125 mg) = 20 mL

Rounding Tip: Liquid medications > 1 mL round to nearest tenth

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Critical Thinking Question

A pediatric client weighing 22 kg is prescribed Ceftriaxone 10 mg/kg once daily. The available Ceftriaxone solution is 100 mg/mL. How many mL should the nurse administer per dose?

1.8 mL
2.2 mL
2.8 mL
3.2 mL

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Body Surface Area Method

Formula

BSA (m²) = √[(height in cm × weight in kg) / 3600]

Calculate BSA in m²
Multiply by drug dose/m²
Convert to final form
Apply rounding rules

Safety Note: BSA dosing crucial for narrow therapeutic index drugs

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Body Surface Area Example

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Critical Thinking Question

A pediatric oncology client with a BSA of 0.6 m² is to receive a chemotherapy drug dosed at 5 mg/m². The drug is available at a concentration of 2 mg/mL. How many mL should the nurse administer?

1.0 mL
1.5 mL
2.0 mL
2.5 mL

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Pediatric Drug Safety

Key Safety Considerations

Children are not small adults
Pediatric medication errors can have severe, even fatal, consequences
Narrow therapeutic windows increase risk
Weight or BSA calculations result in more precise measurements
Different rounding rules (round down)

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Pediatric Drug Safety continued…

Key Safety Considerations

Always verify calculations with colleague
Confirm safe dose ranges
Follow the "Rule of 5"
Document weight in kg only

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Pediatric Dosing Examples

Weight-based example

3-year-old with otitis media
Weight: 20 kg
Order: Amoxicillin 15 mg/kg
Calculation: 20 kg × 15 mg/kg = 300 mg

BSA-based example

5-year-old leukemia client
BSA: 0.8 m²
Adult dose: 25 mg
Child's dose = 25 mg × (0.8 / 1.73) = 11.6 mg

Rounding Tip: For pediatrics, round down to prevent overdose

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Rounding Rules - General Guidelines

Tablets/capsules

Whole numbers unless scored

Scored tablets

Halves or quarters

Controlled/extended release

Never split or crush

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Rounding Rules - General Guidelines

Liquid > 1 mL

Nearest tenth (0.1)

Liquid < 1 mL

Nearest hundredth (0.01)

Drops

Whole numbers only

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Rounding Rules - Special Considerations

Pediatric doses

Round down

High-alert Medications

Follow policies

IV rates (adults)

Nearest whole number

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Rounding Rules - Special Considerations continued…

IV rates (pediatric)

Nearest tenth

Chemotherapy

Institutional protocol

Always document rounding decisions

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Rounding Examples

Basic Formula

Calculated: 12.53 mL → 12.5 mL
Why? Liquid > 1 mL rounds to 1 decimal place

Ratio & Proportion

Calculated: 0.783 mL → 0.78 mL
Why? Liquid < 1 mL rounds to 2 decimal places

Dimensional Analysis

Calculated: 19.26 mL/hr → 19.3 mL/hr
Why? Most IV pumps use 1 decimal place

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Comparing Calculation Methods

Method	Advantages	Disadvantages
Basic Formula	Simple, quick	Limited to simple conversions
Ratio & Proportion	Intuitive, visual	Can be confusing with multiple units
Dimensional Analysis	Works for complex problems, reduces errors	Steeper learning curve
Body Weight	Individualizes dosing	Requires accurate weight
BSA	Precise for critical medications	More complex calculations

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References

Authentic World Ltd. (2020). SafeMedicate Rounding Rules Guidelines. https://safemedicate.com/downloads/resources/rounding.pdf

Hogan, M. (2021). Pediatric Medication Calculations: Comprehensive Review for NCLEX. https://www.happynclex.com/wp-content/uploads/2021/03/Pediatric-Meds-Calculations.pdf

OpenStax. (2024). Nursing Pharmacology. Rice University. https://openstax.org/details/books/pharmacology

SafeMedicate. (2020). safeMedicate Rounding Rules Guidelines. https://safemedicate.com/downloads/resources/rounding.pdf

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