Common Registration Assessment, Part 1

Urine output and fluid balance calculations

Three original questions on urine output thresholds and balance charts, with the working shown and the usual traps pointed out.

Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.

IV drips and monitors on a hospital ward
Photo from Pexels, used under the Pexels licence. It is a stock image and does not show the exercise.

Urine output is assessed in mL per kg per hour. An output below 0.5 mL/kg/h in an adult is oliguria and needs review, so the first step is always to express what you see in those units.

A fluid balance chart adds every input and every output over 24 hours. A positive balance means intake exceeded output.

The method

  1. Convert the measured urine volume to mL per hour using the collection period.
  2. Divide by the weight in kg for mL/kg/h and compare with 0.5.
  3. For a balance chart, total all inputs (oral, IV, feeds) and all outputs (urine, vomit, drains).
  4. Balance = inputs minus outputs; state whether it is positive or negative.
  5. Check your totals column by column before answering.

Three practice questions

Work each one on paper first, then open the answer. All drugs and patients are fictional.

Question 1

A 70 kg patient passed 510 mL of urine in 24 hours. What is the output in mL/kg/h to two decimal places, and is it below the 0.5 mL/kg/h threshold?

Show the answer

Answer: 0.30 mL/kg/h, yes, it is below the threshold

Working: 510 ÷ 24 = 21.25 mL/h. 21.25 ÷ 70 = 0.30 mL/kg/h, which is below 0.5, so the patient is oliguric.

Question 2

Over 24 hours a patient takes 1,500 mL orally and 900 mL intravenously, and passes 1,600 mL of urine, 200 mL of vomit and 150 mL from a drain. What is the fluid balance?

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Answer: Positive 450 mL

Working: Inputs: 1,500 + 900 = 2,400 mL. Outputs: 1,600 + 200 + 150 = 1,950 mL. Balance = 2,400 - 1,950 = +450 mL.

Question 3

An 80 kg patient should produce at least 0.5 mL/kg/h. Over 8 hours they passed 260 mL. How far short are they of the minimum expected volume?

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Answer: 60 mL short

Working: Minimum rate: 0.5 × 80 = 40 mL/h. Over 8 hours that is 320 mL. 320 - 260 = 60 mL short.

How this is examined

Always reduce urine output to mL/kg/h before judging it, and lay balance charts out as two totals before subtracting.

Where marks are lost

  • Comparing the raw 24 hour volume with 0.5 without converting to mL/kg/h.
  • Missing an output line on the chart, usually drains or insensible losses stated in the stem.
  • Reporting the balance sign the wrong way round: positive means more in than out.

Frequently asked questions

Is 0.5 mL/kg/h the only threshold used?

It is the standard adult oliguria threshold used in these questions. Some definitions use 30 mL per hour, which gives a similar answer for average weights.

What counts as an input?

Everything the patient takes in: oral fluids, IV infusions, IV medicines given as volumes, and tube feeds. Include blood products if the stem lists them.

Sources

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

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More calculation topics

See also the formula sheet, the eight sample questions and the approved calculator page.