Common Registration Assessment, Part 1
BMI calculations for the GPhC assessment
Three original questions on body mass index, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Body mass index is weight in kilograms divided by height in metres squared. The formula is one line, but the exam makes you earn it: heights arrive in centimetres or feet and inches, and weights arrive in stones and pounds.
Get everything into kilograms and metres before you divide, and square the height before you touch the weight. Most wrong answers come from dividing by the height instead of its square.
The method
- Write the formula: BMI = weight (kg) ÷ height (m) squared.
- Convert the height to metres: 1 inch = 2.54 cm, 1 foot = 12 inches.
- Convert the weight to kilograms: 1 stone = 14 pounds, 1 pound is about 0.454 kg.
- Square the height first, then divide the weight by that figure.
- Round sensibly and state the band the answer falls in.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A patient weighs 82 kg and is 1.78 m tall. What is their BMI to one decimal place, and which NHS band does it fall in?
Show the answer
Answer: 25.9, in the overweight band
Working: 1.78 squared = 3.1684. 82 ÷ 3.1684 = 25.88, which rounds to 25.9. NHS bands class 25 to 29.9 as overweight.
Question 2
A patient is 5 feet 9 inches tall and weighs 11 stone 6 pounds. Calculate their BMI to one decimal place.
Show the answer
Answer: 23.6
Working: Height: 69 inches × 2.54 = 175.26 cm = 1.7526 m. Weight: 160 pounds × 0.454 = 72.6 kg. 1.7526 squared = 3.0716. 72.6 ÷ 3.0716 = 23.6, inside the healthy weight band of 18.5 to 24.9.
Question 3
A patient is 1.65 m tall and weighs 78 kg. What weight would put them at a BMI of 24.9, and how much would they need to lose?
Show the answer
Answer: 67.8 kg, so a loss of 10.2 kg
Working: 1.65 squared = 2.7225. 24.9 × 2.7225 = 67.8 kg. 78 - 67.8 = 10.2 kg to lose.
How this is examined
Expect at least one question where the units need converting before the formula works. Show every conversion step; method marks are lost when the squared term is skipped.
Where marks are lost
- Dividing by the height instead of the height squared.
- Using centimetres in the formula: 178 instead of 1.78 inflates the answer wildly.
- Treating stones as pounds: 11 stone is 154 pounds before you add the extra pounds.
Frequently asked questions
Do I need to memorise the imperial conversions?
The assessment expects you to work with the units given, so know the common ones: 1 inch = 2.54 cm, 1 stone = 14 pounds, 1 pound is about 0.454 kg.
What are the NHS BMI bands for adults?
Under 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or more is obese. Lower thresholds apply for some ethnic backgrounds.
Sources
The questions above are original and use fictional drugs. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Bioavailability and equivalent oral doses
- Oral maintenance dose and dosing interval
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
- Unit conversions for mass and volume
- Units and units per mL
- Reconstituting powders and final concentration
- Millimoles and milliequivalents
- Infusion time and volume remaining
- Days supply from pack sizes
- Percentage w/v and w/w conversions
- Ideal and adjusted body weight from formulae
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
- Age-band doses from a provided table
- Tablet fractions and split doses
- Stones, pounds, inches and metric conversions
- Micrograms per minute from mg per hour
- Percentage change and mark-up
- Molar solutions and mass per volume
- Osmolarity from component concentrations
- Direct proportion and cross-multiplication
- Scientific notation and very small quantities
- Stock use and reorder quantities
- Parts per million and very dilute solutions
- Sodium chloride equivalents and isotonic solutions
- Density and specific gravity conversions
- Doses per day and hours between doses
- Finding the volume of diluent to add
- Capping a dose at a stated maximum
- Whole vials and ampoules to draw up
- Reading values from a provided graph
- Micrograms per kg per minute infusions
- Percentage composition of a compound preparation
- Suppository displacement values
- Isotonicity by freezing point depression
- Energy content of parenteral nutrition
- Syringe driver rate calculations
- Area under the curve calculations
- Renal dose adjustments using provided bands
- Insulin units and infusion calculations
- Working backwards from a stated dose
- Doses and days from multi-dose devices
- Combined additives in one infusion
- Fewest tablets to make a dose
- Clearance from half-life and volume of distribution
- Maintenance fluid requirements by weight
- Sodium load from effervescent tablets
- Ethanol content of liquid preparations
- Oxygen cylinder duration calculations
- Urine output and fluid balance calculations
- Quantities for titration packs
- Infusion line priming and dead space
See also the formula sheet, the eight sample questions and the approved calculator page.