NMC CBT Drug Calculations: Worked Examples and Practice Questions
Introduction
Part A of the NMC CBT consists of 15 drug calculation questions. It is marked separately from Part B, and you must achieve the required standard in Part A independently. For many internationally educated nurses, this is the most anxiety-provoking section of the exam — particularly for those who qualified several years ago and have not routinely performed manual calculations, or who trained with different dosing systems and units.
The good news: the calculations tested in the NMC CBT follow a small number of standard types. Once you have mastered the formula for each type and practised enough examples, Part A becomes the most reliably scoreable part of the entire exam.
This guide covers the four main calculation types with worked examples for each. Work through every example yourself before checking the solution.
The Four Calculation Types
Type 1: Tablet and Capsule Dose Calculations
These questions give you the prescribed dose and the available tablet or capsule strength, and ask how many tablets or capsules to give.
Formula:
Number of tablets = Prescribed dose ÷ Tablet strength
Example 1
A patient is prescribed paracetamol 1 g orally. The available tablets are paracetamol 500 mg each.
How many tablets should be given?
Working: First, convert to the same units: 1 g = 1,000 mg.
Number of tablets = 1,000 mg ÷ 500 mg = 2 tablets
Example 2
A patient is prescribed metformin 850 mg orally. Available: metformin 500 mg tablets.
How many tablets should be given?
Working: 850 mg ÷ 500 mg = 1.7 tablets
This result should prompt a clinical check — 1.7 tablets is not a standard dosing increment. However, in the context of a CBT calculation question, the arithmetic answer is 1.7. If this type of result appears in the real exam, re-read the question carefully as there may be additional information.
For most CBT questions, the answer will be a whole number or a simple half tablet (0.5). If you are getting an odd fraction, recheck your conversion — a common error is failing to convert grams to milligrams (or vice versa) before calculating.
Example 3
A patient is prescribed codeine phosphate 60 mg orally. Available: codeine phosphate 30 mg tablets.
How many tablets should be given?
Working: 60 mg ÷ 30 mg = 2 tablets
Type 2: Liquid Medication Calculations
These questions give you the prescribed dose, the available stock concentration (amount per volume), and ask how many millilitres to administer.
Formula:
Volume to give (mL) = (Prescribed dose ÷ Stock strength) × Stock volume
Or more simply: Volume to give = (What you want ÷ What you have) × Volume it comes in
Example 4
A patient is prescribed amoxicillin 500 mg orally. Available: amoxicillin 250 mg/5 mL suspension.
How many mL should be given?
Working: Volume = (500 mg ÷ 250 mg) × 5 mL = 2 × 5 mL = 10 mL
Example 5
A child is prescribed ibuprofen 150 mg orally. Available: ibuprofen 100 mg/5 mL suspension.
How many mL should be given?
Working: Volume = (150 mg ÷ 100 mg) × 5 mL = 1.5 × 5 mL = 7.5 mL
Example 6
A patient is prescribed morphine sulphate 10 mg orally. Available: morphine sulphate oral solution 5 mg/5 mL.
How many mL should be given?
Working: Volume = (10 mg ÷ 5 mg) × 5 mL = 2 × 5 mL = 10 mL
Type 3: IV Infusion Rate Calculations
These questions ask you to calculate the rate at which an IV infusion should be administered. The two most common variants are:
- mL/hour: used when a pump is set by the nurse
- drops/minute: used for gravity-fed infusions where you count drops
Formula for mL/hour:
Rate (mL/hour) = Total volume (mL) ÷ Time (hours)
Formula for drops/minute:
Rate (drops/min) = [Volume (mL) × Drop factor (drops/mL)] ÷ Time (minutes)
The standard drop factor for most giving sets is 20 drops/mL. Some blood administration sets use 15 drops/mL. The question will always specify.
Example 7
A patient is prescribed 1 litre (1,000 mL) of 0.9% sodium chloride to be infused over 8 hours via an infusion pump.
At what rate should the pump be set (mL/hour)?
Working: Rate = 1,000 mL ÷ 8 hours = 125 mL/hour
Example 8
A patient requires 500 mL of sodium chloride 0.9% to be administered over 4 hours via an infusion pump.
At what rate should the pump be set (mL/hour)?
Working: Rate = 500 mL ÷ 4 hours = 125 mL/hour
Example 9
A patient is to receive 1 litre (1,000 mL) of 5% dextrose over 6 hours via gravity infusion. The giving set delivers 20 drops/mL.
At what rate in drops per minute should the infusion run?
Working: First convert time to minutes: 6 hours × 60 = 360 minutes.
Rate = (1,000 mL × 20 drops/mL) ÷ 360 minutes = 20,000 ÷ 360 = 55.6 drops/minute
Round to the nearest whole drop: 56 drops/minute
The CBT will typically ask you to round to the nearest whole number for drops/minute calculations. Confirm this in the question.
Example 10
A patient is to receive 250 mL of blood over 2 hours via a blood transfusion giving set (15 drops/mL).
At what rate in drops per minute should the infusion run?
Working: Time in minutes = 2 × 60 = 120 minutes.
Rate = (250 mL × 15 drops/mL) ÷ 120 minutes = 3,750 ÷ 120 = 31.25 drops/minute
Rounded to the nearest whole drop: 31 drops/minute
Type 4: Weight-Based Dosing Calculations
These questions give you a dose expressed per kilogram of body weight and ask you to calculate the total dose or volume for a patient of a specified weight.
Formula:
Total dose = Dose per kg × Patient weight (kg)
Then apply the liquid medication formula if a volume is required.
Example 11
A child weighing 20 kg is prescribed amoxicillin at a dose of 25 mg/kg orally three times daily. Available: amoxicillin 125 mg/5 mL suspension.
What volume should be given for each dose?
Working: Step 1 — Calculate total dose: 25 mg/kg × 20 kg = 500 mg per dose. Step 2 — Calculate volume: (500 mg ÷ 125 mg) × 5 mL = 4 × 5 mL = 20 mL per dose
Example 12
An adult patient weighing 70 kg is prescribed gentamicin at 5 mg/kg IV once daily. Available: gentamicin 40 mg/mL injection.
What volume of gentamicin injection should be drawn up?
Working: Step 1 — Calculate total dose: 5 mg/kg × 70 kg = 350 mg. Step 2 — Calculate volume: (350 mg ÷ 40 mg) × 1 mL = 8.75 mL = 8.75 mL
Key Tips for Part A
Convert units before calculating. The single most common error in drug calculation questions is attempting the calculation in mixed units — for example, dividing milligrams by grams. Always convert to the same unit before you start. The most common conversion is grams to milligrams: 1 g = 1,000 mg.
Write out your working. In the CBT exam, a scratch pad or paper may be available. Use it. Writing each step clearly prevents errors that come from trying to hold numbers in your head.
Check your answer makes clinical sense. If your answer is 12 tablets or 300 mL, something has gone wrong — re-read the question and recheck your conversion. A single dose of 2–4 tablets and volumes of 5–20 mL are typical for oral medications. IV infusion rates are typically 50–200 mL/hour.
Practise without a calculator first. The CBT provides an on-screen calculator, but if you are not comfortable with the arithmetic, relying on the calculator for every step slows you down. Practise until you can perform the basic steps mentally and use the calculator only to verify.
Do not skip Part A preparation. Some candidates spend all their time on Part B content and assume their clinical experience will carry them through the numeracy section. Drug calculation questions in the CBT use specific UK prescribing formats and units. If you have not done this type of calculation recently, dedicated practice is essential.
Practise NMC numeracy and drug calculation questions — work through at least 30 examples before your exam to build the automatic recall this section requires.
Start Practising
The 15 questions in Part A are achievable marks. Unlike the clinical knowledge questions in Part B, there is always a single objectively correct answer to a drug calculation — and you can verify it. Build the habit of checking your answer for clinical plausibility after every calculation, and use the worked examples above as your template for approaching any question type you encounter.
Sign up for free access to the NMC CBT numeracy module and begin your Part A preparation today.