Common Registration Assessment, Part 1
Percentage strength calculations: w/v, w/w and ratios
Three original questions on percentage and ratio strengths, converted to mg per mL before anything else.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
A percentage strength is a quick label: 1% w/v means 1 g in 100 mL. The calculation becomes easy once you turn it into mg per mL, which is the unit most other questions use.
Ratio strengths such as 1 in 1,000 mean 1 g in 1,000 mL. They look different but follow the same rule.
The method
- Write what the strength means in grams per 100 mL (or per 100 g).
- Convert to mg per mL by multiplying by 10.
- Multiply by the volume to find the total amount.
- Convert grams and milligrams as the question asks.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
How many grams of solute are in 250 mL of a 2% w/v solution?
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Answer: 5 g
Working: 2% w/v is 2 g in 100 mL, or 20 mg/mL. 20 × 250 = 5,000 mg = 5 g.
Question 2
A 1 in 5,000 w/v solution is available. How many mg are in 3 mL?
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Answer: 0.6 mg
Working: 1 in 5,000 w/v is 1 g in 5,000 mL = 1,000 mg ÷ 5,000 mL = 0.2 mg/mL. 0.2 × 3 = 0.6 mg.
Question 3
How many mg of drug are needed to make 400 mL of a 0.25% w/v solution?
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Answer: 1,000 mg
Working: 0.25% w/v is 0.25 g in 100 mL = 2.5 mg/mL. 2.5 × 400 = 1,000 mg.
How the definition is stated
The Pharmaceutical Journal's article on molecular weight calculations describes % w/v as the mass of solute in grams per 100 mL of solution. That definition is the whole trick: 2% w/v is 2 g per 100 mL, so 20 mg/mL.
Where marks are lost
- Treating % w/v as g per mL instead of g per 100 mL.
- Losing a factor of 10 or 1,000 moving between g, mg and mL.
- Confusing w/v with w/w; they are different bases.
Frequently asked questions
What does 1% w/v mean?
1 g of solute in 100 mL of solution, which is 10 mg/mL.
What does 1 in 1,000 mean?
1 g in 1,000 mL, which is 1 mg/mL, the same as 0.1% w/v.
Sources
- The Pharmaceutical Journal, medication maths: calculations involving molecular weight (24 March 2025)
- GPhC, Common Registration Assessment specification and permitted items for 2026
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
- 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
See also the formula sheet, the eight sample questions and the approved calculator page.