Units, Concentrations and Conversions
Translate the request into one target unit and safely convert dose and concentration expressions before calculating.
How to prepare for the PSA
Combine patient-centred prescribing judgement with fast BNF navigation, reliable calculations, complete prescriptions and explicit monitoring and communication.
Core concepts
Concept 1
A calculation item asks for one numeric result with the appropriate unit, based on a brief clinical scenario.
Exam cue: Write the requested quantity and target unit before touching the numbers.
Concept 2
Dose and concentration expressions must be converted onto compatible units before multiplication or division.
Exam cue: Convert all inputs to compatible units on separate lines.
Concept 3
Percentages, ratios, mass-per-volume expressions and unit-based medicines require careful interpretation rather than a memorised shortcut.
Exam cue: Carry units through the calculation so cancellation exposes an incorrect setup.
Risk pitfalls and guardrails
Mixing grams, milligrams or micrograms in one operation.
Guardrail: Do not let a familiar medicine, reference search or calculator output replace patient-specific safety checks and escalation.
Treating two differently expressed concentrations as directly comparable.
Guardrail: Do not let a familiar medicine, reference search or calculator output replace patient-specific safety checks and escalation.
Entering a number without the unit or with a unit different from the request.
Guardrail: Do not let a familiar medicine, reference search or calculator output replace patient-specific safety checks and escalation.
Memory anchors
Target Unit
Write the exact unit requested before setting up the calculation.
Compatible Units
Convert dose and concentration into compatible units before dividing.
Unit Cancellation
Carry units through each step and confirm they cancel to the target.
Concentration
Read concentration as an amount of medicine in a stated amount of preparation.
Final Label
Record one clear numeric answer with the requested unit.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
A liquid contains 250 mg in 5 mL. What volume gives 500 mg?
An injection contains 1 mg/mL. What volume contains 500 micrograms?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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