Common Registration Assessment, Part 1
Percentage w/v and w/w conversions
Practise converting percentage strengths into grams and milligrams per mL or per gram, with three original worked questions.
Published 8 October 2026. Independent revision material, not GPhC questions, not for patient care.
A w/v percentage means grams in 100 mL. A w/w percentage means grams in 100 g. Reading the letters correctly tells you the denominator.
The numbers are fictional. The step is the same each time: turn the percentage into an amount per 100, then scale.
The method
- Read whether the strength is w/v or w/w.
- Write the percentage as grams per 100 mL or per 100 g.
- Scale to the volume or weight in the question.
- Convert to mg if the answer asks for it.
- For mg/mL from w/v, multiply the percentage by 10.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
How many grams are in 200 mL of a 0.5% w/v solution?
Show the answer
Answer: 1 g
Working: 0.5 g per 100 mL. For 200 mL, 0.5 × 2 = 1 g.
Question 2
How many grams of drug are in 150 g of a 2% w/w cream?
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Answer: 3 g
Working: 2 g per 100 g. 2 × 1.5 = 3 g.
Question 3
State the strength of a 0.1% w/v solution in mg/mL.
Show the answer
Answer: 1 mg/mL
Working: 0.1 g per 100 mL = 100 mg per 100 mL = 1 mg/mL.
Per 100 anchor
Always anchor the percentage to 100 mL or 100 g before scaling. It prevents most slips.
Where marks are lost
- Treating w/v as w/w.
- Forgetting to convert grams to mg.
- Dividing by 100 twice.
Frequently asked questions
What does w/v stand for?
Weight in volume, usually grams per 100 mL.
Is 1% always 10 mg/mL?
For a w/v solution, yes: 1 g in 100 mL is 10 mg/mL. It does not apply to w/w.
Sources
- The Pharmaceutical Journal, medication maths: dilutions
- The Pharmaceutical Journal, medication maths: using provided formula
- 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
- Ideal and adjusted body weight from formulae
- 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.