PA Drug Calculations 2025 – 12 Exam-Style Dosage & IV Rate Questions (With Answers)

Struggling with drug calculations? This guide gives you a fast formulas cheat-sheet plus 12 exam-style questions with step-by-step solutions—exactly the kind of maths that shows up in PARA KBA and OSCE scenarios. When you’re ready to practise properly, jump into our interactive bank in the web app, start a free trial, and compare your mock percentile with peers.

Try it now: app.matrixeducation.co.uk

Quick Formulas Cheat-Sheet

Dose (mg) = weight (kg) × mg/kg
Volume (mL) = dose (mg) ÷ concentration (mg/mL)
Infusion rate (mL/hr) = total volume (mL) ÷ time (hr)
Drops/min = (mL/hr × drop factor) ÷ 60
%w/v → mg/mL: 1% = 1 g per 100 mL = 10 mg/mL
Dilution: C₁V₁ = C₂V₂
Ratio strengths: 1:1000 adrenaline = 1 mg/mL; 1:10,000 = 0.1 mg/mL

Clinical safety note: Always follow local policies and check the BNF/product literature. These examples are for learning only.

12 Exam-Style Questions with Worked Answers

Q1 · Paeds oral dose (mg/kg): Amoxicillin 15 mg/kg TDS for a 22 kg child. Suspension is 250 mg/5 mL. What volume per dose?

Step 1: Dose = 22 × 15 = 330 mg.
Step 2: 250 mg/5 mL → 50 mg/mL. Volume = 330 ÷ 50 = 6.6 mL per dose.

Q2 · IV pump (mL/hr): Infuse 1 L over 8 hours.

Rate = 1000 ÷ 8 = 125 mL/hr.

Q3 · Drops/min (macrodrip 20 gtt/mL): 500 mL over 4 hours.

mL/hr = 500 ÷ 4 = 125. Drops/min = (125 × 20) ÷ 60 = ≈ 42 gtt/min.

Q4 · Dilution: Prepare 250 mL of 0.9% NaCl from 3% stock. How much stock + diluent?

V₁ = (0.9 × 250) ÷ 3 = 75 mL stock; add 175 mL diluent.

Q5 · Unit conversion: Convert 0.5 mg to micrograms.

0.5 mg = 500 micrograms (mcg).

Q6 · Infusion strength → mL/hr: Morphine 10 mg in 50 mL. Prescribed 2 mg/hr. What pump rate?

Strength = 10 mg ÷ 50 mL = 0.2 mg/mL. Rate = 2 ÷ 0.2 = 10 mL/hr.

Q7 · Paeds paracetamol: 15 mg/kg 6-hourly for 18 kg. Suspension 120 mg/5 mL. Volume per dose?

Dose = 18 × 15 = 270 mg. 120 mg/5 mL → 24 mg/mL. Volume = 270 ÷ 24 = ≈ 11.25 mL.

Q8 · Ratio strength: What is the mg/mL of adrenaline 1:10,000?

0.1 mg/mL.

Q9 · % w/w cream: How much drug in 30 g of a 5% cream?

5% of 30 g = 1.5 g = 1500 mg.

Q10 · Microdrip set (60 gtt/mL): 100 mL over 30 min. Drops/min?

mL/hr = (100 ÷ 0.5) = 200. Drops/min = (200 × 60) ÷ 60 = 200 gtt/min.

Q11 · Convert % to mg/mL: 0.2% lidocaine equals?

0.2% = 0.2 g/100 mL = 2 mg/mL.

Q12 · Paeds dexamethasone: 0.15 mg/kg for 28 kg. Solution 2 mg/5 mL. Volume?

Dose = 28 × 0.15 = 4.2 mg. 2 mg/5 mL → 0.4 mg/mL. Volume = 4.2 ÷ 0.4 = 10.5 mL.

Pro tip: Practise these under time pressure, then switch to mixed SBAs so the maths is embedded inside clinical scenarios.

Build Calculations Into Your 6-Week Plan

Weeks 1–2: 5–10 calc questions/day in tutor mode inside the Matrix web app.
Weeks 3–4: Timed 25-question blocks (45 min), mixing calc + clinical SBAs.
Week 5: Full 200-item mock—use your percentile and domain map to target weak areas.
Week 6: Retake incorrect items only. Keep a one-page formula sheet.