Common Registration Assessment, Part 1
Mass and molar drug concentrations
Some labs report drug levels in micromol/L and some doses are in mg. Three original questions on converting between them.
Published 6 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Pharmaceutical Journal's article on pharmacokinetics adds a molar correction factor for targets given in molar units. Its example is phenytoin, with a molecular weight of 252.3, so 1 mg is about 4 micromoles and a target of 80 micromol/L is the same as 20 mg/L.
The conversion is the same idea as in the molecular weight tutorials: a mole of a substance weighs its molecular weight in grams, so a micromole weighs its molecular weight in micrograms.
The method
- Write the molecular weight in g/mol, which is also micrograms per micromole.
- To go from mg/L to micromol/L, multiply by 1,000 and divide by the molecular weight.
- To go from micromol/L to mg/L, multiply by the molecular weight and divide by 1,000.
- Use the mg/L figure in dose = volume × concentration.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A target phenytoin level is 20 mg/L and the molecular weight is 252.3. What is that in micromol/L, to one decimal place?
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Answer: 79.3 micromol/L
Working: 20 mg/L = 20,000 micrograms/L. 20,000 ÷ 252.3 = 79.27 micromol/L, which is 79.3 to one decimal place. The journal rounds this to 80 for practical use.
Question 2
A drug with a molecular weight of 180 has a target level of 45 micromol/L. What is that in mg/L? These figures are made up for the exercise.
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Answer: 8.1 mg/L
Working: 45 micromol/L × 180 micrograms per micromole = 8,100 micrograms/L, which is 8.1 mg/L.
Question 3
A drug with a molecular weight of 200 has a target level of 100 micromol/L and a volume of distribution of 30 L. What loading dose in mg is needed? These figures are made up for the exercise.
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Answer: 600 mg
Working: 100 micromol/L × 200 = 20,000 micrograms/L = 20 mg/L. Dose = 30 L × 20 mg/L = 600 mg.
What the article says
The article describes the molar correction factor and shows that 80 micromol/L of phenytoin equals 20 mg/L. The questions here use different drugs and numbers.
Where marks are lost
- Dividing by 1,000 when you should multiply, or the reverse.
- Forgetting that mg/L and micrograms/L differ by a factor of 1,000.
- Using the dose in mg with a concentration still in micromol/L.
Frequently asked questions
How do I check the answer is sensible?
For a drug with a molecular weight below 1,000, the number in micromol/L is larger than the number in mg/L, so check your answer goes that way.
Where do I find the molecular weight?
The question gives it, or it comes from a permitted reference.
Sources
- The Pharmaceutical Journal, back to basics: pharmacokinetics
- GPhC, Common Registration Assessment specification and permitted items for 2026
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
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
See also the formula sheet, the eight sample questions and the approved calculator page.