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Calculation Skills · PSA section guide

Calculation Skills in the PSA: what it tests, how it is marked, and a real question

Work out a dose, a volume, a rate or a dilution from the printed strength and the patient, and enter one number in the unit the box asks for.

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8items in the exam
2marks each
16 / 200of the paper
93questions in the bank

At a glance

Items in the exam8
Marks per item2
Share of the paper16 of 200
Question styleType a number in the stated unit
Primary skillDose, volume, rate and conversion arithmetic
BNF useUsually the question supplies the numbers; the BNF confirms the dose where required
PassthePSA bank93 questions, rated easy to hard
On PassthePSAThe same numeric box with the unit shown, working in the explanation
Time at exam pace1 min 12 s

What this section is really testing

  • Reading a preparation's strength correctly: mg per mL, micrograms per dose, units per mL, percentage weight per volume.
  • Turning a weight-based or per-day dose into the single figure asked for: one dose, one volume, one rate.
  • Converting between units without losing a factor of a thousand.
  • Sanity-checking the magnitude against what a real patient would receive.

A repeatable approach

  1. Read the unitsIn the question, on the preparation, and in the answer box. Write them down before touching the calculator.
  2. Identify the targetOne dose? A daily dose? A volume? A rate per hour? The wrong target is the commonest error.
  3. Calculate the doseWeight times dose per kilogram, or the stated dose, in mass or units.
  4. Convert to what is givenDose divided by concentration gives volume; volume over time gives rate.
  5. Check the magnitudeWould a nurse draw up this volume? Would this rate empty the bag in a sensible time?

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BNF strategy for this section

The exam is open book, and the time is short. Know which heading answers which question before you open the monograph.

Question values→Dose per kg or per dose→Preparation strength→Convert units→Answer in the box's unit
Is the dose per kg correct?
Indications and dose, under the matching indication and age band
What strength does it come in?
Medicinal forms
Is there a maximum?
Indications and dose, the maximum stated for the indication
Renal dose reduction?
Renal impairment

The general technique, with drills, is in the BNF speed guide.

The formulas, and one worked example each

Dose from weightdose = dose per kg × weight15 mg/kg for a 24 kg child: 15 × 24 = 360 mg.
Single dose from a daily dosesingle dose = daily dose ÷ doses per day18 mg a day in two divided doses: 18 ÷ 2 = 9 mg per dose.
Volume from concentrationvolume = required dose ÷ concentration9 mg needed from a 3.75 mg/mL solution: 9 ÷ 3.75 = 2.4 mL.
Infusion raterate (mL/h) = total volume ÷ hours1000 mL over 8 hours: 1000 ÷ 8 = 125 mL/h.
Percentage strengthX % = X g in 100 mL = X × 10 mg/mLLidocaine 1%: 10 mg/mL, so 5 mL contains 50 mg.
Ratio strength1 in N = 1 g in N mLAdrenaline 1 in 1000: 1 mg/mL. 1 in 10 000: 0.1 mg/mL, so 100 micrograms/mL.
Unit conversions1 g = 1000 mg 1 mg = 1000 micrograms 1 L = 1000 mLMove the decimal three places; write the unit on every line.
DilutionC1 × V1 = C2 × V210 mL of a 10 mg/mL solution to 100 mL gives 1 mg/mL.

Unit check

ggrams
mgmilligrams, ×1000 smaller
micrograms×1000 smaller again; write it in full
mLa volume, never a dose
unitsinsulin, heparin: not convertible to mass
The thousand

Every calculation item is one decimal place away from a thousandfold error. 0.5 mg and 500 micrograms are the same; 500 mg is a different drug. Write micrograms in full, as the BNF does, and convert before you divide.

Common traps

Daily versus single doseThe regimen says 18 mg a day in two doses; the question asks for one dose. 9 mg, not 18.
Micrograms and milligramsMicrograms to milligrams and back. 500 micrograms is 0.5 mg; writing 500 mg is a thousandfold overdose.
Percent means g per 100 mLA 1% solution is 1 g in 100 mL, so 10 mg per mL. A 1 in 1000 solution is 1 g in 1000 mL, so 1 mg per mL.
Rounding earlyCarry the full figure through and round once at the end, to the precision the box asks for.
Answering in the wrong unitThe box says mL and you typed mg. Right arithmetic, zero marks.

Areas worth being comfortable with

Not predictions. These are the broad clinical areas this section draws on, taken from the shape of the exam and the BNF; being fluent in them makes the lookup fast.

  • Weight-based doses in adults and children
  • Volumes from mg per mL preparations
  • Infusion rates in mL per hour and drops per minute
  • Percentage and ratio strengths: 1%, 1 in 1000, 1 in 10 000
  • Units: insulin, heparin, some antibiotics
  • Dividing a daily dose into a regimen
  • Oral liquid volumes for paediatric doses
  • Simple dilutions

A real calculation skills question from the bank

This is one of the 20 items every visitor can try free, in the interface the exam uses. Work it before you open the answer.

ID : CAL-48Calculation Skills2 marksmedium
Case presentation

A 6-year-old boy weighing 22 kg is nil by mouth after open reduction of a forearm fracture and needs regular analgesia. Intravenous paracetamol is prescribed at 15 mg/kg per dose. The preparation available on the ward is a vial of paracetamol 10 mg/mL solution for infusion, and the nurse asks what volume to draw up for one dose.

CalculationWhat volume (mL) of the 10 mg/mL solution provides one 15 mg/kg dose?
(write your answer in the box provided)
AnswermL
Answer and marking
Correct answer

33 mL

Marks

2

Explanation

Step 1: dose = 15 mg/kg × 22 kg = 330 mg. Step 2: volume = 330 mg ÷ 10 mg/mL = 33 mL.

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Before you submit

  • Did I answer in the unit the box asks for?
  • One dose or the daily dose?
  • Micrograms and milligrams: did the thousand go the right way?
  • Did I round only at the end?
  • Is the magnitude plausible for a real patient?

Practice questions

93 Calculation Skills questions, easy to hard, marked like the exam

Every answer comes with the explanation and the BNF reference behind it. Twenty questions across all eight sections are free. Pass guarantee: fail after 400+ questions and your access is extended free for 3 months.

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Continue revising

PrescribingChoosing the right drug, then writing it safely10 marks per item →Data InterpretationActing on results: continue, adjust, hold or stop2 marks per item →Drug MonitoringKnowing what to measure, when, and what to do with it2 marks per item →

View all 8 sections and the exam overview →

Section weightings follow the published PSA blueprint. The BNF headings named here are the current online BNF's. Check any dose or threshold against the BNF before you rely on it; this page teaches the method, the BNF holds the numbers.

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