Dose, Quantity and Patient-Size Calculations
Calculate an administrable dose or quantity from prescribed dose, available strength, weight or body surface area.
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
Patient-size calculations translate a dose per kilogram or body surface area into a total dose before selecting an administrable quantity.
Exam cue: Calculate the total required dose before converting it into tablets or volume.
Concept 2
Tablet, capsule, liquid and other formulation calculations must use the available strength and any clinically appropriate rounding rule.
Exam cue: Keep the patient-size value and dose-rate unit paired correctly.
Concept 3
The result should be checked against patient age, formulation limits and the magnitude expected from the source information.
Exam cue: Round only at the clinically appropriate stage and verify the available formulation.
Risk pitfalls and guardrails
Using pounds as kilograms or confusing body weight with body surface area.
Guardrail: Do not let a familiar medicine, reference search or calculator output replace patient-specific safety checks and escalation.
Rounding an intermediate value so early that the final dose changes materially.
Guardrail: Do not let a familiar medicine, reference search or calculator output replace patient-specific safety checks and escalation.
Reporting the calculated dose when the task asks for tablets, volume or total quantity.
Guardrail: Do not let a familiar medicine, reference search or calculator output replace patient-specific safety checks and escalation.
Memory anchors
Total Dose First
Convert a per-size instruction into the patient's total dose before finding quantity.
Available Strength
Use the actual amount of medicine in each tablet, capsule or volume.
Requested Quantity
Answer in dose, tablets, volume or duration exactly as requested.
Rounding Point
Round only when needed for safe practical administration.
Magnitude Check
Compare the result with the patient, formulation and expected clinical scale.
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 18-kg child needs 15 mg/kg per dose. What dose is required?
A 72-kg adult needs 0.1 unit/kg/hour insulin. What rate is required?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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