CBT Numeracy: Drug Calculation Methods for the NMC Test

CBT Part A tests 15 questions on numeracy applied to nursing. The breadth covers everything from a simple tablet dose to a weight-based infusion calculation. This chapter walks through each method.

The actual exam uses UK clinical conventions. Mass in mg, mcg, g. Volume in mL, L. Time in minutes or hours. Standard drip set factors (20 drops/mL for clear fluids, 15 drops/mL for blood, 60 drops/mL for micro sets) as appropriate.

Method 1: Tablet and capsule calculations

The basic dose-volume problem.

Formula: number of tablets = (dose required) ÷ (dose per tablet)

Worked example: prescription for 750 mg paracetamol. Available: 500 mg tablets.

• 750 ÷ 500 = 1.5 tablets.

The CBT will typically structure tablet questions so the answer is a whole or half tablet (.5). Questions that produce awkward fractions usually indicate either a misread or a unit mismatch. Recheck.

Method 2: Liquid oral medication

Same formula extended to volume.

Formula: volume required = (dose required ÷ dose per unit volume) × unit volume

Or more practically: volume = (what you want ÷ what you’ve got) × the volume it’s in.

Worked example: prescription for 250 mg amoxicillin oral syrup. Available: 125 mg/5 mL.

• (250 ÷ 125) × 5 = 10 mL.

Method 3: IV drip rate (drops per minute)

For gravity-fed IV using a drip set.

Formula: drops per minute = (volume to be infused × drip set factor) ÷ time in minutes

Drip set factors used in the CBT:

• Standard clear fluid set: 20 drops per mL.
• Blood set: 15 drops per mL.
• Micro/paediatric set: 60 drops per mL.

Worked example: 1,000 mL normal saline over 8 hours using a standard set.

• Time in minutes: 8 × 60 = 480.
• Drops per minute: (1,000 × 20) ÷ 480 = 41.67 ≈ 42 drops/min.

The CBT usually expects rounding to the nearest whole drop or half drop.

Method 4: IV infusion rate (mL per hour)

For pump-controlled infusion.

Formula: mL per hour = total volume ÷ time in hours

Worked example: 500 mL over 4 hours.

• 500 ÷ 4 = 125 mL/hour.

For continuous infusions calibrated to a drug dose:

Formula: mL per hour = (dose required × volume of solution) ÷ amount of drug in solution

Worked example: heparin infusion of 1,000 units/hour. Solution: 25,000 units in 500 mL.

• (1,000 × 500) ÷ 25,000 = 20 mL/hour.

Method 5: Weight-based dosing

Used for paediatric medications and some adult drugs.

Formula: dose = (dose per kg × patient weight in kg)

Worked example: gentamicin 5 mg/kg for a 60 kg adult.

• 5 × 60 = 300 mg.

The trap on weight-based questions is the unit. Always check the question is asking for the dose, the volume, or the rate, and what units those should be in.

Method 6: Unit conversions

The conversions used most commonly:

• 1 g = 1,000 mg
• 1 mg = 1,000 mcg
• 1 mcg = 1,000 nanograms
• 1 L = 1,000 mL

Worked example: prescription for 0.25 g paracetamol. Available 250 mg tablets.

• Convert: 0.25 g = 250 mg.
• Tablets: 250 ÷ 250 = 1 tablet.

The CBT will sometimes deliberately mix units in the question, for example giving a prescription in g and an available stock in mg, to test that you convert before calculating.

Method 7: Fluid balance

Sum of fluids in vs fluids out.

Worked example: A patient has 1,500 mL oral intake, 1,000 mL IV intake, 1,200 mL urine output, 200 mL drain output, 800 mL insensible losses (estimated).

• Total in = 2,500 mL.
• Total out = 2,200 mL.
• Balance = +300 mL (positive balance).

The CBT may also test percentage dehydration or rehydration calculations using weight loss.

Method 8: BMI

Body Mass Index calculation.

Formula: BMI = weight in kg ÷ (height in metres)²

Worked example: patient 70 kg, 1.65 m tall.

• 1.65² = 2.7225
• 70 ÷ 2.7225 = 25.7 BMI.

BMI categories the CBT may ask about:

• Under 18.5: underweight.
• 18.5-24.9: healthy range.
• 25-29.9: overweight.
• 30+: obese.

The single most common error pattern

Across all methods, the error pattern that costs candidates most marks is unit mismatch. A calculation done correctly but with the answer in the wrong unit scores zero.

Two habits that prevent this:

1. Convert units before calculating rather than at the end.
2. Sanity-check the answer: does this look like a reasonable nursing dose, rate or volume? A calculated dose of 5,000 tablets or 0.01 mL is almost certainly an error.

The next chapter has 10 worked CBT-style numeracy examples covering each of these methods.