Common Registration Assessment, Part 1
Reading values from a provided graph
Practise taking exact values and rates of change from a graph supplied in the question, with three original worked questions.
Published 10 October 2026. Independent revision material, not GPhC questions, not for patient care.
Some questions draw a level-time graph or a calibration line and ask you to read from it. The marks go to careful reading: which axis, which gridline, and which point the question means.
The graphs described here are fictional and given in words so you can practise the arithmetic. In the assessment, read the scale twice before writing anything down.
The method
- Check what each axis shows and the value of each gridline.
- Find the point or crossing the question asks about.
- Read across to the axis and write the value with its unit.
- For a rate of change, use two clear points: change in level divided by change in time.
- Use the read values in any further calculation exactly as read.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional provided graph shows a drug level of 12 mg/L at 1 hour, falling by half every 4 hours. What level does the graph show at 5 hours?
Show the answer
Answer: 6 mg/L
Working: From 1 hour to 5 hours is one 4-hour halving period. 12 / 2 = 6 mg/L.
Question 2
A fictional provided graph shows a level of 9 mg/L at 3 hours and 4.5 mg/L at 6 hours. What is the average fall per hour over that interval?
Show the answer
Answer: 1.5 mg/L per hour
Working: Fall = 9 - 4.5 = 4.5 mg/L over 6 - 3 = 3 hours. 4.5 / 3 = 1.5 mg/L per hour.
Question 3
A fictional provided graph has horizontal gridlines every 2 mg/L. A plotted point sits one gridline below 10 mg/L. What level does it show?
Show the answer
Answer: 8 mg/L
Working: One gridline below 10 mg/L is 10 - 2 = 8 mg/L.
Annotate the axes
Write the value of one gridline next to each axis before reading any point. It takes seconds and removes the commonest reading error.
Where marks are lost
- Reading the wrong axis or assuming each gridline is one unit.
- Estimating between gridlines when the point sits exactly on one.
- Using a level from the wrong time point in the follow-on calculation.
Frequently asked questions
How exact do graph readings need to be?
Questions are designed so the answer falls on a clear gridline or crossing. If your reading is awkward, re-read the scale before going further.
What if the question asks for a rate of change?
Pick the two clearest points on the line, subtract the levels, divide by the time between them, and keep the unit as level per hour.
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
- 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
- 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.