Common Registration Assessment, Part 1
Ideal and adjusted body weight from formulae
Practise using supplied weight formulae and then applying a per-kg dose, with three fictional worked questions.
Published 8 October 2026. Independent revision material, not GPhC questions, not for patient care.
Some questions give a formula for ideal or adjusted weight and then ask for a dose per kg. The formulae here are fictional and stated in full, as an exam question would state them.
Do the weight calculation first, keep the unrounded value, then apply the dose. This is arithmetic practice, not a method for any real drug.
The method
- Write the supplied formula exactly as given.
- Substitute the values and calculate the weight.
- Check the weight is plausible against the stated actual weight.
- Apply the dose per kg to the correct weight.
- Round only as the question instructs.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional adjusted weight is ideal weight + 0.4 × (actual − ideal). Ideal is 60 kg and actual is 90 kg. Find the adjusted weight.
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Answer: 72 kg
Working: 90 − 60 = 30. 0.4 × 30 = 12. 60 + 12 = 72 kg.
Question 2
A fictional ideal weight formula is 50 + 0.9 × (height in cm − 152). Height is 172 cm. Find the ideal weight.
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Answer: 68 kg
Working: 172 − 152 = 20. 0.9 × 20 = 18. 50 + 18 = 68 kg.
Question 3
A fictional dose is 5 mg/kg using adjusted weight. Use 72 kg. Find the dose.
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Answer: 360 mg
Working: 5 × 72 = 360 mg.
Brackets first
Work the bracket before multiplying. Write each step on its own line.
Where marks are lost
- Using actual weight when the question says adjusted weight.
- Doing 0.4 × actual instead of 0.4 × (actual − ideal).
- Rounding the weight before applying the dose.
Frequently asked questions
Do I need to know these formulae?
Use the formula in the question. Do not rely on remembered ones.
Why keep the unrounded weight?
Early rounding can change the final dose.
Sources
- The Pharmaceutical Journal, medication maths: using provided formula
- The Pharmaceutical Journal, preparing for the GPhC pharmaceutical calculations assessment
- GPhC, 2026 assessment specification and permitted items
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
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
See also the formula sheet, the eight sample questions and the approved calculator page.