Common Registration Assessment, Part 1
Insulin units and infusion calculations
Three original questions on insulin units, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Insulin is prescribed in units, never milligrams, and the arithmetic is about converting between units, volumes and time. A standard strength of 100 units per mL keeps most questions to one division.
The questions here are fictional and practise only the arithmetic. Write the strength as a single line, 100 units = 1 mL, and derive everything else from it.
The method
- Write the strength, for example 100 units per mL.
- Convert the prescribed units to a volume, or the volume to units.
- For an infusion, divide units per hour by units per mL for the mL rate.
- For a device, divide the total units by the daily use for the days covered.
- Give whole days only when the question asks how long supplies last.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional infusion contains 50 units of insulin made up to 50 mL. The prescription is 4 units per hour. What pump rate in mL per hour is needed?
Show the answer
Answer: 4 mL/hour
Working: Strength = 50 / 50 = 1 unit per mL. 4 units / 1 unit per mL = 4 mL/hour.
Question 2
A fictional bag contains 100 units in 100 mL, running at 6 units per hour. How many units and how many mL are delivered over 8 hours?
Show the answer
Answer: 48 units, 48 mL
Working: Strength = 1 unit per mL. 6 x 8 = 48 units, which is 48 mL at this strength.
Question 3
A fictional insulin pen holds 3 mL at 100 units per mL. The patient uses 18 units per day. For how many full days does one pen last?
Show the answer
Answer: 16 days
Working: Total = 3 x 100 = 300 units. 300 / 18 = 16.7 days, so 16 full days.
One anchor line
Write the strength as units equals mL at the top of your rough work. Every insulin question is one multiplication or division away from that line.
Where marks are lost
- Treating units as mg; insulin questions never need a mass conversion.
- Dividing the wrong way: units per hour divided by units per mL gives mL per hour.
- Rounding 16.7 days up to 17 when the question asks for full days.
Frequently asked questions
Is it always 100 units per mL?
No. Other strengths exist, so read the stem. The method is identical whatever the printed strength.
Why whole days?
A part day is not a full day's cover. When the question asks how long a device lasts, count only complete days.
Sources
- The Pharmaceutical Journal, medication maths: how to solve dose and dosage calculations (video)
- 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
- 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
- Suppository displacement values
- Isotonicity by freezing point depression
- Energy content of parenteral nutrition
- Syringe driver rate calculations
- Area under the curve calculations
- Renal dose adjustments using provided bands
- Working backwards from a stated dose
- Doses and days from multi-dose devices
- Combined additives in one infusion
See also the formula sheet, the eight sample questions and the approved calculator page.