Common Registration Assessment, Part 1
Syringe driver rate calculations
Three original questions on syringe driver rates, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
A syringe driver pushes a small volume over a long period, usually 24 hours. Rates are set in millilitres per hour or in millimetres of syringe scale per hour, and the two are linked by the syringe's dimensions.
The questions here are fictional and practise only the arithmetic. Keep the volume, the scale length and the time in front of you, and convert between them one step at a time.
The method
- Write the total volume and the total time.
- Divide volume by time for mL per hour.
- If the driver works in mm, divide the syringe scale length by the time instead.
- To link the two, divide the volume by the scale length to get mL per mm.
- Multiply a mm rate by mL per mm to check the dose rate in mL or mg per hour.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional syringe contains 17 mL to be delivered over 24 hours. What rate in mL per hour is set, to one decimal place?
Show the answer
Answer: 0.7 mL/hour
Working: 17 / 24 = 0.708 mL/hour, which rounds to 0.7 mL/hour.
Question 2
The same syringe has a usable scale length of 48 mm and contains 12 mL. It is set to run over 24 hours. What is the rate in mm per hour, and how much solution is delivered per hour?
Show the answer
Answer: 2 mm/hour, 0.5 mL/hour
Working: 48 / 24 = 2 mm/hour. 12 mL / 48 mm = 0.25 mL per mm. 2 x 0.25 = 0.5 mL/hour.
Question 3
A fictional syringe driver is running at 1.5 mm/hour on a 60 mm syringe containing 18 mL. How long will the syringe last, and what is the rate in mL per hour?
Show the answer
Answer: 40 hours, 0.45 mL/hour
Working: 60 / 1.5 = 40 hours. 18 / 40 = 0.45 mL/hour.
Volume, scale, time
Every syringe driver question is built from three numbers. Write them in a line and convert one pair at a time.
Where marks are lost
- Mixing up mm per hour and mL per hour; they are only equal by coincidence.
- Using the full syringe barrel length instead of the usable scale length given.
- Rounding too early when converting between mm and mL.
Frequently asked questions
Why do some drivers use mm per hour?
Some models advance the plunger by distance, so the prescription is written against the syringe scale. The exam tells you which unit the driver uses.
Does the drug strength change the rate?
No. The rate comes from volume, scale and time. Strength only matters when you convert the rate into a dose per hour.
Sources
- The Pharmaceutical Journal, medication maths: infusion rates
- 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
- Area under the curve calculations
- Renal dose adjustments using provided bands
- Insulin units and infusion calculations
- 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.