Common Registration Assessment, Part 1
Unit conversions for mass and volume
Practise moving between micrograms, milligrams and grams, and between millilitres and litres, with three original worked questions.
Published 8 October 2026. Independent revision material, not GPhC questions, not for patient care.
Many calculation errors start before any dosing arithmetic: the numbers are right but the units do not match. Each step down the mass ladder from grams to milligrams to micrograms multiplies by 1000. Each step up divides by 1000.
Write the unit beside every number and convert once, at the start, so that the rest of the working sits in one unit. The questions below use fictional drugs and are arithmetic practice only.
The method
- Underline the unit the answer must be in.
- Convert every quantity to a common unit before calculating.
- Use factors of 1000: 1 g = 1000 mg and 1 mg = 1000 micrograms.
- Carry units through each line of working.
- Check the size of the answer against the quantities you started with.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
Convert 0.25 mg to micrograms.
Show the answer
Answer: 250 micrograms
Working: Going down one step multiplies by 1000: 0.25 × 1000 = 250 micrograms.
Question 2
A fictional dose is 1.5 g. Tablets contain 500 mg each. How many tablets make the dose?
Show the answer
Answer: 3 tablets
Working: 1.5 g = 1500 mg. 1500 ÷ 500 = 3 tablets.
Question 3
A fictional solution contains 2.4 g in 800 mL. State the concentration in mg/mL.
Show the answer
Answer: 3 mg/mL
Working: 2.4 g = 2400 mg. 2400 ÷ 800 = 3 mg/mL.
Size check
If you converted to a smaller unit, the number should get bigger. If you converted to a larger unit, it should get smaller. A quick glance catches most direction errors.
Where marks are lost
- Moving the factor of 1000 in the wrong direction.
- Mixing mg and micrograms in one line.
- Converting the answer at the end instead of the inputs at the start.
Frequently asked questions
Why convert at the start?
It keeps every line in one unit, so a slip is easier to spot and the final answer needs no further change.
Can I use powers of ten?
Yes. 0.25 mg = 0.25 × 10^3 micrograms. Use whichever form you can check quickly.
Sources
- The Pharmaceutical Journal, medication maths: quantities to supply and using provided formulae
- 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
- 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
See also the formula sheet, the eight sample questions and the approved calculator page.