Common Registration Assessment, Part 1
Micrograms per kg per minute infusions
Practise converting a micrograms per kg per minute rate into mL per hour for a stated concentration, with three original worked questions.
Published 10 October 2026. Independent revision material, not GPhC questions, not for patient care.
Critical care style rates are given per kg per minute, while pumps are set in mL per hour. The conversion chain has four links: multiply by the weight, convert minutes to hours, convert micrograms to the concentration unit, then divide by the concentration.
Doing the chain in a fixed order beats trying to hold it in your head. The examples are fictional and practise the chain only.
The method
- Multiply the rate by the weight to get micrograms per minute.
- Multiply by 60 to get micrograms per hour.
- Work out the concentration in micrograms per mL.
- Divide micrograms per hour by the concentration to get mL per hour.
- Keep every unit written down through the chain.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional infusion runs at 5 micrograms/kg/min for a 70 kg patient. The bag contains 50 mg in 250 mL. What rate in mL per hour delivers this?
Show the answer
Answer: 105 mL per hour
Working: 5 x 70 = 350 mcg/min. x 60 = 21,000 mcg/hr. Concentration = 50,000 mcg / 250 mL = 200 mcg/mL. Rate = 21,000 / 200 = 105 mL/hr.
Question 2
A fictional infusion runs at 0.1 micrograms/kg/min for an 80 kg patient. The bag contains 1 mg in 250 mL. What is the pump rate in mL per hour?
Show the answer
Answer: 120 mL per hour
Working: 0.1 x 80 = 8 mcg/min. x 60 = 480 mcg/hr. Concentration = 1000 mcg / 250 mL = 4 mcg/mL. Rate = 480 / 4 = 120 mL/hr.
Question 3
A fictional infusion of 25 mg in 500 mL runs at 30 mL per hour for a 75 kg patient. What rate is this in micrograms per kg per minute?
Show the answer
Answer: 0.33 micrograms/kg/min
Working: Concentration = 25,000 mcg / 500 mL = 50 mcg/mL. Delivery = 30 x 50 = 1500 mcg/hr = 25 mcg/min. Per kg = 25 / 75 = 0.33 mcg/kg/min.
Chain in one order
Weight, then 60, then concentration, then divide. Writing the same four headings for every question of this type makes the arithmetic routine.
Where marks are lost
- Skipping the x 60 step, so the rate is out by a factor of 60.
- Using mg per mL as the concentration while the rate is in micrograms.
- Dividing by the weight instead of multiplying at the first step.
Frequently asked questions
Why convert minutes to hours at all?
Pumps are programmed in mL per hour, so the rate has to end in hours. Skipping that step is the commonest factor-of-60 error in this topic.
Can I work backwards from mL per hour?
Yes. Multiply the pump rate by the concentration to get mcg per hour, divide by 60, then divide by the weight.
Sources
- The Pharmaceutical Journal, back to basics: pharmacokinetics
- 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
- Reading values from a provided graph
- Percentage composition of a compound preparation
See also the formula sheet, the eight sample questions and the approved calculator page.