Common Registration Assessment, Part 1
Ferrous sulfate drops and tablet calculations
Three original questions on ferrous sulfate drops and 200 mg tablets, using the NHS adult doses.
Published 13 October 2026. Independent revision material, not GPhC questions, not for patient care.
The NHS page gives ferrous sulfate as 200 mg tablets, or drops containing 125 mg of ferrous sulfate in 1 mL. The adult dose to treat anaemia is one 200 mg tablet once to three times a day, or 4 mL of drops once or twice a day.
To prevent anaemia the dose is one 200 mg tablet daily, or 2.4 mL to 4.8 mL of drops daily. The calculations convert between drops in mL and mg, or work out how many tablets or days of supply a prescription needs.
The method
- Read the strength: 125 mg per mL for the drops, 200 mg for each tablet.
- To get mg from drops, multiply the volume in mL by 125.
- To get a volume from mg, divide the mg by 125.
- For tablets, multiply tablets per dose by doses per day by days.
- Compare tablet and drops regimens in mg of ferrous sulfate, not in numbers of units.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
An adult is told to take 4 mL of ferrous sulfate drops (125 mg per mL) twice a day. How many mg is one dose, and how many mg in a day?
Show the answer
Answer: 500 mg per dose, 1,000 mg a day
Working: 125 × 4 = 500 mg. 500 × 2 = 1,000 mg.
Question 2
To prevent anaemia an adult takes 2.4 mL a day of the drops. How many mg is the smallest and the largest NHS preventive dose (2.4 mL and 4.8 mL), and how many days does a 100 mL bottle last at 4 mL a day?
Show the answer
Answer: 300 mg and 600 mg, and 25 days
Working: 125 × 2.4 = 300 mg. 125 × 4.8 = 600 mg. 100 ÷ 4 = 25 days.
Question 3
An adult is prescribed one 200 mg tablet three times a day for 28 days. How many tablets are needed, and what volume of 125 mg per mL drops would give the same daily mg?
Show the answer
Answer: 84 tablets, and 4.8 mL a day
Working: 3 × 28 = 84 tablets. Daily mg: 3 × 200 = 600 mg. 600 ÷ 125 = 4.8 mL.
Exam approach
Convert everything to mg of ferrous sulfate. Then tablets and drops can be compared directly, and the supply is a simple multiplication.
Where marks are lost
- Comparing 4 mL of drops with one tablet as though they were the same amount.
- Using 200 mg per mL, the tablet strength, for the drops.
- Forgetting the number of doses per day.
- Leaving out the unit when comparing a volume with a mass.
Frequently asked questions
Does it matter what ferrous sulfate is taken with?
The NHS page says it works best on an empty stomach, and not to take it with tea, coffee, eggs or dairy, leaving a 2 hour gap. A vitamin C source may help absorption.
What if the tablets upset the stomach?
The NHS page says the doctor may advise alternate days, and that it can be taken with or after food if it upsets the stomach.
Sources
- NHS, how and when to take ferrous sulfate
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional patients and prescriptions. 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
- Insulin units and infusion calculations
- Working backwards from a stated dose
- Doses and days from multi-dose devices
- Combined additives in one infusion
- BMI calculations for the GPhC assessment
- Fewest tablets to make a dose
- Clearance from half-life and volume of distribution
- Maintenance fluid requirements by weight
- Sodium load from effervescent tablets
- Ethanol content of liquid preparations
- Oxygen cylinder duration calculations
- Urine output and fluid balance calculations
- Quantities for titration packs
- Infusion line priming and dead space
- Alcohol unit calculations for the GPhC assessment
- Paracetamol syrup calculations for children
- Ibuprofen liquid calculations for children
- Vitamin D conversions for the GPhC assessment
- Amoxicillin liquid and capsule calculations
- Folic acid dose and quantity calculations
- Adult paracetamol and the daily maximum
- Calcium and iron intake calculations
- Salt limit calculations by age
See also the formula sheet, the eight sample questions and the approved calculator page.