ExamFinch.com My Study

Study guide

4.2 Pharmacy Calculations

Practice this exam

PTCE Pharmacy Technician Certification Exam

Track your progress as you read

Finished sections get a check mark.

1 Introduction

About the PTCE How to Use This Guide

2 Chapter 1: Medications

1.1 Generic and Brand Drug Names & Classifications 1.2 Cardiovascular and Anticoagulant Medications 1.3 Anti-Infective and Analgesic Medications 1.4 Endocrine, Respiratory, CNS, and Storage Requirements

3 Chapter 2: Federal Requirements

2.1 DEA Regulations and Controlled Substance Schedules 2.2 Drug Supply Chain Security Act (DSCSA) 2.3 FDA Regulations and Drug Recalls 2.4 HIPAA, CMEA, and Prescription Requirements

4 Chapter 3: Patient Safety and Quality Assurance

3.1 Medication Error Prevention 3.2 High-Alert and Look-Alike/Sound-Alike Medications 3.3 Patient Counseling and Communication 3.4 Quality Assurance and Continuous Quality Improvement

5 Chapter 4: Order Entry and Processing

4.1 Prescription Interpretation 4.2 Pharmacy Calculations 4.3 Inventory Management and NDC Numbers 4.4 Insurance, Billing, and Prescription Processing

4.2 Pharmacy Calculations

Key Takeaways Explain This Topic AI

Day supply calculation: (Quantity dispensed / Daily dose) = Days supply.

Dosage calculations require matching units and using dimensional analysis.

Concentration conversions: 1% = 1g/100mL = 10mg/mL.

Ratio strength: 1:1000 means 1g of drug per 1000mL of solution.

Common conversions: 1 tsp = 5mL, 1 tbsp = 15mL, 1 oz = 30mL, 1 kg = 2.2 lb.

Last updated: November 2026

Quick Answer: Pharmacy calculations include day supply (quantity divided by daily dose), dosage calculations (using dimensional analysis), and concentration conversions (1% = 10mg/mL). These calculations are heavily tested on the PTCE and are essential for daily pharmacy work.

Pharmacy calculations are among the most practical skills tested on the PTCE. This section covers day supply, dosage calculations, concentration conversions, and the essential conversion factors you must know.

  • Essential Conversion Factors
  • Volume Conversions
  • Conversion Value
  • 1 teaspoon (tsp) 5 mL
  • 1 tablespoon (tbsp) 15 mL (3 tsp)
  • 1 fluid ounce (fl oz) 30 mL (2 tbsp)
  • 1 cup 240 mL (8 fl oz)
  • 1 pint 480 mL (16 fl oz)
  • 1 quart 960 mL (32 fl oz)
  • 1 gallon 3,840 mL (128 fl oz)
  • 1 liter (L) 1,000 mL
  • Weight Conversions
  • Conversion Value
  • 1 kilogram (kg) 1,000 grams (g)
  • 1 gram (g) 1,000 milligrams (mg)
  • 1 milligram (mg) 1,000 micrograms (mcg)
  • 1 pound (lb) 454 grams
  • 1 kilogram (kg) 2.2 pounds
  • 1 ounce (oz) 28.4 grams
  • 1 grain (gr) 65 mg (approximately)
  • Day Supply Calculations

Day supply tells insurance companies and patients how long a prescription will last.

  • Basic Formula Day Supply = Quantity Dispensed / (Dose per administration × Administrations per day)
  • Or simply: Day Supply = Quantity Dispensed / Daily Dose
  • Example 1: Tablets
  • Rx: Lisinopril 10mg #90, Sig: 1 tab PO QD

Calculation:

  • Quantity = 90 tablets
  • Daily dose = 1 tablet/day
  • Day supply = 90 ÷ 1 = 90 days
  • Example 2: Multiple Daily Doses
  • Rx: Metformin 500mg #180, Sig: 1 tab PO BID

Calculation:

  • Quantity = 180 tablets
  • Daily dose = 2 tablets/day (BID = twice daily)
  • Day supply = 180 ÷ 2 = 90 days
  • Example 3: Liquid Medications
  • Rx: Amoxicillin 250mg/5mL, Disp: 150mL, Sig: 1 tsp TID

Calculation:

  • Quantity = 150 mL
  • Each dose = 1 tsp = 5 mL
  • Doses per day = 3 (TID)
  • Daily usage = 5 mL × 3 = 15 mL/day
  • Day supply = 150 ÷ 15 = 10 days
  • Example 4: Eye/Ear Drops
  • Rx: Latanoprost 0.005% ophthalmic solution, 2.5mL, Sig: 1 gtt OU QHS
  • Standard Estimate: 20 drops per mL (industry standard for most ophthalmic solutions)

Calculation:

  • Total drops = 2.5 mL × 20 drops/mL = 50 drops
  • Drops per day = 1 drop × 2 eyes = 2 drops/day
  • Day supply = 50 ÷ 2 = 25 days
  • Example 5: Inhalers
  • Rx: Albuterol HFA 90mcg inhaler, 200 actuations, Sig: 2 puffs Q4-6H PRN

PRN Calculation (Maximum Daily Use):

  • Maximum frequency: every 4 hours = 6 times/day
  • Puffs per use: 2
  • Maximum daily puffs = 2 × 6 = 12 puffs/day
  • Day supply = 200 ÷ 12 = 16.7 = 16 days (round down for PRN)

Note: For PRN medications, calculate based on maximum frequency to determine minimum day supply.

  • Dosage Calculations
  • Weight-Based Dosing

Many pediatric and some adult medications are dosed based on weight.

Formula: Dose = Weight (kg) × Dose per kg

Example: A child weighs 44 lbs. The prescribed dose is 25 mg/kg/day in divided doses TID. What is each dose?

  • Step 1: Convert weight to kg
  • 44 lbs ÷ 2.2 = 20 kg
  • Step 2: Calculate total daily dose
  • 20 kg × 25 mg/kg = 500 mg/day
  • Step 3: Divide into three doses (TID)
  • 500 mg ÷ 3 = 166.7 mg per dose (round to 167 mg)
  • BSA-Based Dosing (Body Surface Area)

Some medications (often chemotherapy) are dosed based on BSA.

  • Mosteller Formula: BSA (m²) = √[(Height (cm) × Weight (kg)) / 3600]
  • Concentration Calculations
  • Percentage Strength
  • Expression Meaning
  • % w/v (weight/volume) Grams per 100 mL
  • % v/v (volume/volume) mL per 100 mL
  • % w/w (weight/weight) Grams per 100 grams
  • Key Conversion: 1% = 1g/100mL = 10mg/mL

Example: How many mg of hydrocortisone are in 30g of 2.5% hydrocortisone cream?

Calculation:

  • 2.5% = 2.5g per 100g
  • 30g of cream × (2.5g/100g) = 0.75g = 750 mg
  • Ratio Strength

Ratio strength expresses concentration as a ratio.

  • Ratio Meaning
  • 1:100 1g in 100mL (or 1g per 100g)
  • 1:1000 1g in 1000mL
  • 1:10,000 1g in 10,000mL

Example: How many mg of epinephrine are in 5mL of a 1:1000 solution?

Calculation:

  • 1:1000 = 1g per 1000mL = 1mg/mL
  • 5mL × 1mg/mL = 5 mg
  • Converting Between Ratio and Percentage Percentage = (1 / Ratio denominator) × 100
  • Example: Convert 1:10,000 to a percentage
  • (1/10,000) × 100 = 0.01%
  • Dilution Calculations

Basic Dilution Formula (C1V1 = C2V2) Initial Concentration × Initial Volume = Final Concentration × Final Volume

Example: How much stock 70% isopropyl alcohol is needed to make 500mL of 30% solution?

Calculation:

C1 = 70%, V1 = ?

  • C2 = 30%, V2 = 500mL
  • 70% × V1 = 30% × 500mL
  • V1 = (30 × 500) / 70 = 214.3 mL

Mix 214.3 mL of 70% alcohol with 285.7 mL of water (500 - 214.3) to make 500mL of 30% solution.

Alligation Method

Alligations were removed from the 2026 PTCE outline; this method is background only.

Used when mixing two different strengths to achieve a desired concentration.

Example: Mix 20% and 5% solutions to make 100mL of 12% solution. How much of each is needed?

  • Alligation Grid: 20% | 7 parts (12 - 5 = 7)
  • X
  • 12% (desired)
  • X
  • 5% | 8 parts (20 - 12 = 8)

Correct Method:

  • Parts of 20% needed: |12 - 5| = 7 parts
  • Parts of 5% needed: |20 - 12| = 8 parts
  • Total parts = 7 + 8 = 15 parts

For 100mL:

  • Volume of 20%: (7/15) × 100 = 46.7 mL
  • Volume of 5%: (8/15) × 100 = 53.3 mL