Common Registration Assessment, Part 1
Parts per million and very dilute solutions
Practise converting between parts per million, percentage strength and mass per volume, with three original worked questions.
Published 10 October 2026. Independent revision material, not GPhC questions, not for patient care.
Very dilute solutions are easier to express in parts per million than as a percentage. For water-based liquids, 1 ppm is 1 mg of solute in 1 litre, because 1 litre of water weighs about 1 kg.
The questions here are fictional and practise only the conversions. Fix one anchor in your head, 1 ppm = 1 mg/L, and derive everything else from it.
The method
- Write the anchor: 1 ppm = 1 mg per litre for aqueous liquids.
- Convert the given amount to mg and the volume to litres.
- Divide mg by litres to get ppm, or multiply ppm by litres to get mg.
- To go from a percentage to ppm, convert the percentage to mg per litre first.
- Check the unit the question wants before you answer.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional solution contains 4 mg of solute made up to 2 litres with water. What is the concentration in ppm?
Show the answer
Answer: 2 ppm
Working: 4 mg in 2 L = 2 mg per litre, and 1 mg/L = 1 ppm, so 2 ppm.
Question 2
A fictional disinfectant solution is 250 ppm. How many mg of active substance are in 500 mL?
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Answer: 125 mg
Working: 250 ppm = 250 mg per litre. In 0.5 L: 250 x 0.5 = 125 mg.
Question 3
A fictional solution is 0.002% w/v. Express this in ppm.
Show the answer
Answer: 20 ppm
Working: 0.002% w/v = 0.002 g in 100 mL = 2 mg in 100 mL = 20 mg in 1 litre = 20 ppm.
Anchor every answer
Write 1 ppm = 1 mg/L at the top of your rough work before you start. Every conversion in this topic is one multiplication or division away from that line.
Where marks are lost
- Treating 1 ppm as 1 mg per mL instead of 1 mg per litre.
- Forgetting to convert mL to litres before dividing.
- Moving the decimal point the wrong way when starting from a percentage.
Frequently asked questions
Is 1 ppm always 1 mg per litre?
For dilute water-based liquids, yes, because a litre weighs about a kilogram. For dense or non-aqueous liquids the conversion needs the density.
How does ppm relate to percentage strength?
1% w/v is 1 g in 100 mL, which is 10,000 mg per litre, so 1% w/v = 10,000 ppm. Divide the percentage by 0.0001 or multiply the ppm by 0.0001 to swap.
Sources
- The Pharmaceutical Journal, medication maths: dilutions
- 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
- 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
- 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
See also the formula sheet, the eight sample questions and the approved calculator page.