Common Registration Assessment, Part 1
Units and units per mL
Practise doses written in units, with strengths in units per mL, using three fictional examples with worked answers.
Published 8 October 2026. Independent revision material, not GPhC questions, not for patient care.
Some products are measured in units rather than milligrams. The strength is then written as units per mL, and the arithmetic is the same as for mg per mL: volume = dose ÷ strength.
The products here are fictional, and the numbers are not real dosing. The aim is to practise reading a strength and choosing the volume or total units without changing the unit by accident.
The method
- Write the dose in units and the strength in units per mL.
- Divide the dose by the strength to find the volume.
- To find total units, multiply strength by volume.
- Read the vial label for the volume in the container, not just the strength.
- Check that the volume is measurable with the syringe named in the question.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional product has strength 100 units/mL. The dose is 35 units. What volume is needed?
Show the answer
Answer: 0.35 mL
Working: 35 ÷ 100 = 0.35 mL.
Question 2
A fictional vial has 5000 units in 5 mL. The dose is 2500 units. What volume is drawn up?
Show the answer
Answer: 2.5 mL
Working: Strength = 5000 ÷ 5 = 1000 units/mL. 2500 ÷ 1000 = 2.5 mL.
Question 3
A syringe holds 0.8 mL of a 500 units/mL solution. How many units does it contain?
Show the answer
Answer: 400 units
Working: 500 × 0.8 = 400 units.
Read the label twice
Strength and total content often appear close together on a label. Name each one in your working before you divide.
Where marks are lost
- Using the total units in a vial as the strength.
- Dividing strength by dose instead of dose by strength.
- Rounding the volume before the question says to.
Frequently asked questions
Is a unit the same as a milligram?
No. A unit measures biological activity and is specific to the product, so never convert between units and mg unless the question supplies a factor.
Why find units per mL first?
A per-mL strength makes the volume a single division and removes the risk of dividing by the container size.
Sources
- The Pharmaceutical Journal, medication maths: using provided formula
- 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
- 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.