Common Registration Assessment, Part 1
Maintenance fluid requirements by weight
Three original questions on the 100/50/20 rule, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Holliday-Segar method estimates daily maintenance fluid from weight: 100 mL/kg for the first 10 kg, 50 mL/kg for the next 10 kg, and 20 mL/kg for every kilogram above 20 kg.
NICE guidance on intravenous fluid therapy in children describes this method and caps routine maintenance at 2,500 mL per day for males and 2,000 mL per day for females.
The method
- Split the weight into bands: first 10 kg, next 10 kg, the rest.
- Apply 100, 50 and 20 mL/kg to each band and add them for the daily total.
- Divide by 24 if the question wants an hourly rate.
- Check the answer against the NICE caps of 2,500 mL (male) and 2,000 mL (female) per day.
- If the cap bites, state the capped figure, not the raw calculation.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A child weighs 14 kg. What is the daily maintenance fluid requirement and the hourly rate?
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Answer: 1,200 mL per day, 50 mL per hour
Working: First 10 kg: 10 × 100 = 1,000 mL. Next 4 kg: 4 × 50 = 200 mL. Total 1,200 mL per day. 1,200 ÷ 24 = 50 mL per hour.
Question 2
A child weighs 25 kg. Calculate the daily maintenance volume and the hourly rate to the nearest mL.
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Answer: 1,600 mL per day, 67 mL per hour
Working: 1,000 + 500 + (5 × 20) = 1,600 mL per day. 1,600 ÷ 24 = 66.7, which is 67 mL per hour to the nearest mL.
Question 3
A 70 kg woman's maintenance is calculated with the same formula. What volume should be prescribed per NICE guidance, and why?
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Answer: 2,000 mL per day, because the NICE cap for females applies
Working: The formula gives 1,000 + 500 + (50 × 20) = 2,500 mL, but NICE caps routine maintenance at 2,000 mL per day for females, so 2,000 mL is prescribed.
How this is examined
Band the weight before multiplying, and say the units out loud: the question may ask for mL per day or mL per hour, and the two differ by a factor of 24.
Where marks are lost
- Applying 100 mL/kg to the whole weight instead of banding it.
- Forgetting the 20 mL/kg band starts only above 20 kg.
- Quoting the raw formula figure when the NICE male or female cap is lower.
Frequently asked questions
Does this cover deficits or ongoing losses?
No. This is routine maintenance only. Deficits, resuscitation and replacement of abnormal losses are calculated separately.
What are the exact NICE caps?
NICE NG29 limits routine maintenance to 2,500 mL per day for males and 2,000 mL per day for females.
Sources
- NICE NG29, intravenous fluid therapy in children and young people in hospital
- GPhC, Common Registration Assessment specification and permitted items for sittings in 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
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Bioavailability and equivalent oral doses
- Oral maintenance dose and dosing interval
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
- Unit conversions for mass and volume
- Units and units per mL
- Reconstituting powders and final concentration
- Millimoles and milliequivalents
- Infusion time and volume remaining
- Days supply from pack sizes
- Percentage w/v and w/w conversions
- Ideal and adjusted body weight from formulae
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
- Age-band doses from a provided table
- Tablet fractions and split doses
- Stones, pounds, inches and metric conversions
- Micrograms per minute from mg per hour
- Percentage change and mark-up
- Molar solutions and mass per volume
- Osmolarity from component concentrations
- Direct proportion and cross-multiplication
- Scientific notation and very small quantities
- Stock use and reorder quantities
- Parts per million and very dilute solutions
- Sodium chloride equivalents and isotonic solutions
- Density and specific gravity conversions
- Doses per day and hours between doses
- Finding the volume of diluent to add
- Capping a dose at a stated maximum
- Whole vials and ampoules to draw up
- Reading values from a provided graph
- Micrograms per kg per minute infusions
- Percentage composition of a compound preparation
- Suppository displacement values
- Isotonicity by freezing point depression
- Energy content of parenteral nutrition
- Syringe driver rate calculations
- Area under the curve calculations
- Renal dose adjustments using provided bands
- Insulin units and infusion calculations
- Working backwards from a stated dose
- Doses and days from multi-dose devices
- Combined additives in one infusion
- BMI calculations for the GPhC assessment
- Fewest tablets to make a dose
- Clearance from half-life and volume of distribution
- Sodium load from effervescent tablets
- Ethanol content of liquid preparations
- Oxygen cylinder duration calculations
- Urine output and fluid balance calculations
- Quantities for titration packs
- Infusion line priming and dead space
See also the formula sheet, the eight sample questions and the approved calculator page.