Pharmaceutical Calculations Practice Problems for PLE Philippines 2026

By LisensyaPrep Team | Last Updated: May 2026 | 11-minute read

Pharmaceutical calculations are among the most predictable questions in the PLE. Unlike conceptual questions where multiple interpretations are possible, calculation questions have one correct numerical answer. This makes them free points for examinees who practice them consistently.

The key is not memorizing formulas in isolation but understanding what each formula is solving for. This reviewer walks through the major calculation types with worked examples.

Concentration Expressions

Understanding concentration expressions is the foundation of all pharmaceutical calculations.

Dosage Calculations

Dose Based on Body Weight

Formula: Dose = Patient weight (kg) × Dose per kg

Example: A patient weighs 60 kg. The prescribed dose of amoxicillin is 25 mg/kg/day in 3 divided doses. What is the single dose?

Total daily dose = 60 kg × 25 mg/kg = 1,500 mg/day

Single dose = 1,500 mg ÷ 3 = 500 mg per dose

Dose Based on Body Surface Area (BSA)

BSA is used for chemotherapy and pediatric dosing where weight-based calculations are insufficient.

Mosteller formula: BSA (m²) = √(Height cm × Weight kg ÷ 3,600)

Example: Patient height 170 cm, weight 65 kg.

BSA = √(170 × 65 ÷ 3,600) = √(3.069) = 1.75 m²

Dose = BSA × Dose per m²

Pediatric Dosing: Clark's Rule

Clark's Rule uses body weight to estimate pediatric dose from adult dose.

Formula: Child dose = (Child's weight in kg ÷ 70) × Adult dose

Example: Adult dose of paracetamol is 500 mg. Child weighs 20 kg.

Child dose = (20 ÷ 70) × 500 mg = 0.286 × 500 = 143 mg

Young's Rule (Age-Based)

Formula: Child dose = [Age in years ÷ (Age + 12)] × Adult dose

Example: Child is 6 years old. Adult dose is 500 mg.

Child dose = [6 ÷ (6 + 12)] × 500 = (6 ÷ 18) × 500 = 0.333 × 500 = 167 mg

Dilution Calculations

Simple Dilution: C1V1 = C2V2

This is the most used calculation in pharmacy. When you dilute a solution, the amount of solute stays the same but the volume increases.

Formula: C1 × V1 = C2 × V2

Where C1 = initial concentration, V1 = initial volume, C2 = final concentration, V2 = final volume.

Example: How many mL of a 10% solution is needed to prepare 500 mL of a 2% solution?

C1 × V1 = C2 × V2

10% × V1 = 2% × 500 mL

V1 = (2 × 500) ÷ 10 = 100 mL

Add 100 mL of the 10% solution to enough diluent to make 500 mL total.

Alligation Method

Used when mixing two concentrations to get a desired intermediate concentration.

Example: How many mL of 70% alcohol and 30% alcohol are needed to make 1,000 mL of 50% alcohol?

Step 1: Set up the alligation grid.

Step 2: Calculate volumes.

IV Flow Rate Calculations

Drops per Minute (gtt/min)

Formula: Flow rate (gtt/min) = [Volume (mL) × Drop factor (gtt/mL)] ÷ Time (minutes)

Standard drop factors:

Example: Infuse 1,000 mL D5W over 8 hours using a macrodrip set with 20 gtt/mL.

Time in minutes = 8 × 60 = 480 minutes

Flow rate = (1,000 × 20) ÷ 480 = 20,000 ÷ 480 = 41.7 gtt/min ≈ 42 gtt/min

mL per Hour

Formula: Flow rate (mL/hr) = Volume (mL) ÷ Time (hours)

Example: Infuse 500 mL over 4 hours.

Flow rate = 500 ÷ 4 = 125 mL/hr

Percentage Strength Calculations

Calculating Amount of Drug in a Preparation

Formula: Amount of drug = % strength × Volume (or weight) of preparation

Example: How many grams of sodium chloride are in 500 mL of 0.9% NaCl solution?

Amount = 0.9% × 500 mL = 0.009 × 500 = 4.5 g

Calculating Percentage Strength

Formula: % strength = (Amount of drug ÷ Total volume or weight) × 100

Example: 2 g of drug dissolved in 50 mL of solution. What is the % w/v?

% w/v = (2 g ÷ 50 mL) × 100 = 4% w/v

Isotonicity Calculations

Sodium Chloride Equivalent Method

Isotonic solutions have the same osmotic pressure as blood (approximately 0.9% NaCl). Formulations for ophthalmic, nasal, and parenteral use should ideally be isotonic.

Sodium chloride equivalent (E value): The weight of NaCl that is osmotically equivalent to 1 g of the drug.

Formula: w = 0.009 × V − (drug amount × E value)

Where w = weight of NaCl to add, V = volume of solution in mL.

Example: How much NaCl is needed to make 100 mL of a 1% pilocarpine HCl solution isotonic? (E value of pilocarpine HCl = 0.24)

NaCl needed to make 100 mL isotonic = 0.009 × 100 = 0.9 g

NaCl equivalent of drug = 1 g × 0.24 = 0.24 g

Additional NaCl needed = 0.9 − 0.24 = 0.66 g

Powder Volume Calculations

When a dry powder is reconstituted, the powder itself occupies volume. This powder volume must be accounted for in calculations.

Formula: Powder volume = Final volume − Volume of diluent added

Example: A vial of amoxicillin powder is reconstituted by adding 9 mL of water to get 10 mL of final solution. What is the powder volume?

Powder volume = 10 mL − 9 mL = 1 mL

Using powder volume to find concentration: If the vial contains 500 mg of amoxicillin and the final volume is 10 mL:

Concentration = 500 mg ÷ 10 mL = 50 mg/mL

Practice What You Just Learned

Pharmaceutical calculation questions in the PLE are pure points if you practice them. Head to LisensyaPrep and practice the calculation-based questions now. No account needed.

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