Pharmacy Calculations

Quick Answer: The three most common pharmacy calculations are day supply (quantity / daily dose), dosage (desired / available x quantity), and concentrations (C1V1 = C2V2). Master unit conversions and always double-check your math.

Pharmacy calculations are one of the most heavily tested areas on the ExCPT. You must be comfortable with day supply, dosages, concentrations, and unit conversions.

Essential Conversion Factors

Volume Conversions

Weight Conversions

Day Supply Calculations

Day supply tells you how long a medication will last. This is critical for insurance billing and determining refill dates.

Formula

Day Supply = Total Quantity Dispensed / Daily Dose

Worked Example 1: Tablets

Rx: Lisinopril 10mg #90, Sig: i tab PO QD

Step 1: Calculate daily dose

• Patient takes 1 tablet once daily = 1 tablet/day

Step 2: Calculate day supply

• Day Supply = 90 tablets / 1 tablet per day = 90 days

Worked Example 2: Twice Daily Dosing

Rx: Metformin 500mg #60, Sig: i tab PO BID

Step 1: Calculate daily dose

• Patient takes 1 tablet twice daily = 2 tablets/day

Step 2: Calculate day supply

• Day Supply = 60 tablets / 2 tablets per day = 30 days

Worked Example 3: Liquid Medication

Rx: Amoxicillin 250mg/5mL, 150mL, Sig: 5mL PO TID

Step 1: Calculate daily dose

• Patient takes 5 mL three times daily = 15 mL/day

Step 2: Calculate day supply

• Day Supply = 150 mL / 15 mL per day = 10 days

Worked Example 4: Eye Drops

Rx: Latanoprost 0.005% ophthalmic solution, 2.5mL, Sig: 1 gtt OU QHS

Note: Standard assumption is 20 drops per mL for ophthalmic solutions

Step 1: Calculate total drops

• 2.5 mL x 20 drops/mL = 50 drops

Step 2: Calculate daily dose

• 1 drop in both eyes once daily = 2 drops/day

Step 3: Calculate day supply

• Day Supply = 50 drops / 2 drops per day = 25 days

Dosage Calculations

Formula

Amount to Give = (Desired Dose / Available Dose) x Quantity

Worked Example 5: Oral Liquid Dosing

Order: Amoxicillin 400 mg PO Available: Amoxicillin 250 mg/5 mL

Step 1: Set up the calculation

• Amount = (400 mg / 250 mg) x 5 mL

Step 2: Calculate

• Amount = 1.6 x 5 mL = 8 mL

Worked Example 6: Injectable Medication

Order: Morphine 8 mg IM Available: Morphine 10 mg/mL vial

Step 1: Set up the calculation

• Amount = (8 mg / 10 mg) x 1 mL

Step 2: Calculate

• Amount = 0.8 x 1 mL = 0.8 mL

Worked Example 7: Weight-Based Dosing

Order: Amoxicillin 25 mg/kg/day in 2 divided doses Patient weight: 44 lbs Available: Amoxicillin 125 mg/5 mL

Step 1: Convert weight to kg

• 44 lbs / 2.2 = 20 kg

Step 2: Calculate total daily dose

• 25 mg/kg x 20 kg = 500 mg/day

Step 3: Calculate dose per administration

• 500 mg / 2 doses = 250 mg per dose

Step 4: Calculate volume per dose

• (250 mg / 125 mg) x 5 mL = 10 mL per dose

Concentration and Dilution Calculations

Dilution Formula

C1 x V1 = C2 x V2

Where:

• C1 = Initial concentration
• V1 = Initial volume
• C2 = Final concentration
• V2 = Final volume

Worked Example 8: Stock Solution Dilution

How much 95% alcohol is needed to prepare 500 mL of 70% alcohol?

Step 1: Identify variables

• C1 = 95%, V1 = ?
• C2 = 70%, V2 = 500 mL

Step 2: Solve for V1

• 95% x V1 = 70% x 500 mL
• V1 = (70 x 500) / 95
• V1 = 368.4 mL of 95% alcohol

Step 3: Calculate diluent needed

• 500 mL - 368.4 mL = 131.6 mL of water

Percentage Strength

Worked Example 9: Percentage Calculation

How many grams of hydrocortisone are in 60 g of 2.5% hydrocortisone cream?

Step 1: Understand the percentage

• 2.5% w/w means 2.5 g per 100 g

Step 2: Calculate

• (2.5 g / 100 g) x 60 g = 1.5 g of hydrocortisone

Ratio and Proportion

Worked Example 10: Insulin Dosing

How many units of insulin are in 0.3 mL of U-100 insulin?

Note: U-100 means 100 units per mL

Step 1: Set up proportion

• 100 units / 1 mL = x units / 0.3 mL

Step 2: Solve

• x = 100 x 0.3 = 30 units

Common Calculation Pitfalls