Common Registration Assessment, Part 1
Infusion dose per kg per hour
From mL/hour to a weight-based dose and back again. Three original questions with the working shown.
Published 5 October 2026. Independent revision material, not GPhC questions, not for patient care.
Paediatric infusion questions often give a syringe made up to a stated concentration, a rate in mL/hour and a weight, then ask for the dose in units/kg/hour. The Pharmaceutical Journal's dose and dosage tutorial includes a heparin example of exactly this kind.
Convert the syringe to a concentration per mL first. Then the rate gives the amount per hour, and dividing by the weight gives the dose per kg.
The method
- Work out the concentration in units or micrograms per mL.
- Multiply by the rate in mL/hour for the amount per hour.
- Divide by the weight in kg for the dose per kg per hour.
- To go the other way, multiply dose by weight, then divide by the concentration.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A heparin infusion is 5,000 units in 50 mL and runs at 0.4 mL/hour. The patient weighs 12.5 kg. What is the dose in units/kg/hour?
Show the answer
Answer: 3.2 units/kg/hour
Working: 5,000 ÷ 50 = 100 units/mL. 0.4 × 100 = 40 units/hour. 40 ÷ 12.5 = 3.2 units/kg/hour.
Question 2
A drug is prescribed at 20 micrograms/kg/hour for a patient weighing 70 kg. The syringe contains 2 mg in 50 mL. What rate in mL/hour is needed?
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Answer: 35 mL/hour
Working: 20 × 70 = 1,400 micrograms/hour. 2 mg in 50 mL = 2,000 micrograms ÷ 50 = 40 micrograms/mL. 1,400 ÷ 40 = 35 mL/hour.
Question 3
A syringe holds 600 units in 30 mL and runs at 1.5 mL/hour for a child weighing 8 kg. What is the dose in units/kg/hour?
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Answer: 3.75 units/kg/hour
Working: 600 ÷ 30 = 20 units/mL. 1.5 × 20 = 30 units/hour. 30 ÷ 8 = 3.75 units/kg/hour.
The journal's heparin example
In the tutorial, a 3-year-old weighing 13 kg receives prophylactic heparin at 0.5 mL/hour from a syringe of 13,000 units in 50 mL, and the question asks for units/kg/hour. The questions on this page use different values.
Where marks are lost
- Dividing by weight before finding the amount per hour.
- Using the syringe total as if it were per mL.
- Mixing mg and micrograms in the concentration.
Frequently asked questions
Why work per mL first?
It turns every later step into one multiplication or division, and it is easy to check.
Does the same method work for units and micrograms?
Yes. Keep the same unit all the way through and convert only at the end.
Sources
- The Pharmaceutical Journal, medication maths: how to solve dose and dosage calculations (video tutorial, updated 25 March 2025)
- 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
- Rounding doses to measurable volumes
See also the formula sheet, the eight sample questions and the approved calculator page.