Common Registration Assessment, Part 1
Density and specific gravity conversions
Practise converting between mass and volume for liquids using a stated density or specific gravity, with three original worked questions.
Published 10 October 2026. Independent revision material, not GPhC questions, not for patient care.
Some liquids are measured by weight even though the answer is needed as a volume, or the other way round. The link between them is density: mass = density x volume.
Specific gravity is the density relative to water, so a specific gravity of 1.25 means 1 mL weighs 1.25 g. The question always states the figure; your job is to multiply or divide the right way.
The liquids here are fictional and the numbers are chosen to practise the method.
The method
- Write the relationship: mass = specific gravity x volume, in grams and mL.
- To find mass from volume, multiply.
- To find volume from mass, divide.
- Keep units in g and mL throughout, converting at the end if asked.
- Sanity check: a specific gravity above 1 means the liquid is heavier than water.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional liquid has a specific gravity of 1.25. What is the mass of 200 mL?
Show the answer
Answer: 250 g
Working: Mass = 1.25 x 200 = 250 g.
Question 2
A fictional syrup has a specific gravity of 1.32. What volume contains 66 g?
Show the answer
Answer: 50 mL
Working: Volume = 66 / 1.32 = 50 mL.
Question 3
100 mL of a fictional liquid weighs 85 g. What volume holds 170 g?
Show the answer
Answer: 200 mL
Working: Specific gravity = 85 / 100 = 0.85. Volume = 170 / 0.85 = 200 mL.
Write the triangle
Write mass = SG x volume before touching the calculator. Deciding multiply or divide while looking at that line is far safer than deciding in your head.
Where marks are lost
- Dividing when you should multiply, because the relationship was not written down first.
- Mixing up g and kg or mL and L part way through.
- Assuming the liquid weighs the same as water when the specific gravity is not 1.
Frequently asked questions
Is specific gravity the same as density?
For practical purposes with water as the reference, a specific gravity of 1.25 means a density of 1.25 g/mL. You can use them interchangeably in these calculations.
When does this come up in the assessment?
When a formula gives a liquid by weight but you must measure a volume, or when a % w/v strength involves a liquid ingredient weighed rather than measured.
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
- Parts per million and very dilute solutions
- Sodium chloride equivalents and isotonic solutions
- 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.