Common Registration Assessment, Part 1
Ratio strengths and one in X
Convert clearly defined one-in-X strengths to mg/mL and percentage strengths, with three worked questions.
Published 7 October 2026. Independent revision material, not GPhC questions, not for patient care.
A ratio strength is only useful when its basis is clear. For a 1 in 1,000 w/v preparation, this page means 1 g in a final volume of 1,000 mL. That differs from a parts instruction about how much diluent to add.
Write the definition before converting. For 1 in X w/v, mg/mL = 1,000 ÷ X and percentage w/v = 100 ÷ X. A w/w preparation uses grams of final preparation instead of millilitres; do not quietly treat it as w/v.
The method
- Confirm whether the strength is w/v, w/w or v/v.
- For these w/v examples, write 1 g in X mL final volume.
- Convert 1 g to 1,000 mg if mg/mL is wanted.
- Scale to 100 mL if percentage w/v is wanted.
- Use the converted strength for the requested mass or volume.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A solution is 1 in 2,000 w/v, defined as 1 g in 2,000 mL. Express its strength in mg/mL.
Show the answer
Answer: 0.5 mg/mL
Working: 1 g = 1,000 mg. 1,000 ÷ 2,000 = 0.5 mg/mL.
Question 2
Convert 1 in 400 w/v to percentage w/v.
Show the answer
Answer: 0.25% w/v
Working: 1 g in 400 mL is 0.25 g in 100 mL, or 0.25% w/v.
Question 3
A 1 in 5,000 w/v solution is defined as 1 g in 5,000 mL. How much drug in mg is present in 150 mL?
Show the answer
Answer: 30 mg
Working: Strength = 1,000 ÷ 5,000 = 0.2 mg/mL. Amount = 150 × 0.2 = 30 mg.
A denominator check
A larger X means a weaker solution when the numerator stays at 1 g. A 1 in 5,000 solution is weaker than a 1 in 500 solution. Check that direction before accepting a conversion.
Where marks are lost
- Reading one in X as one part plus X parts water.
- Dropping the w/v or w/w basis.
- Using 100 rather than 1,000 to convert grams to milligrams.
Frequently asked questions
Does 1 in 1,000 w/v mean 1 mg/mL?
Yes, under the stated definition: 1 g in 1,000 mL is 1,000 mg in 1,000 mL.
Can I compare w/w and w/v directly?
Not generally. One is mass per mass, the other mass per volume. Further information, such as density, may be needed.
Sources
- The Pharmaceutical Journal, medication maths: dilutions
- 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
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
See also the formula sheet, the eight sample questions and the approved calculator page.