If you are registering as a Child Nurse (RNC) with the NMC, your Part A Numeracy and Part B Clinical sections will feature paediatric-specific calculations. Children have highly sensitive metabolic rates and body-water compositions, making precise dosage critical. Master these three essential formulas to pass your exam with confidence.
1. Weight-Based Dosage Calculation
Paediatric medication is almost always prescribed based on body weight (e.g., mg/kg). The NMC CBT tests this in two steps. First, calculate the total daily dose: Prescribed Dose (mg/kg) × Patient Weight (kg). Second, divide that daily total by the number of doses per day before applying the universal formula: (What you want / What you have) × Volume.
2. The Holiday-Segar Formula (Fluid Maintenance)
You must know how to calculate a child's 24-hour maintenance fluid requirement based on their weight:
- First 10 kg: 100 mL per kg (e.g., a 9kg baby needs 900 mL/day).
- Next 10 kg (11-20 kg): Add 50 mL per kg for each kg over 10.
- Each kg over 20 kg: Add 20 mL per kg for each kg over 20.
The CBT will often ask you to calculate the hourly rate. Take your 24-hour fluid total and divide by 24 to get the exact mL/hour rate.
3. Pediatric Fluid Deficit
For dehydrated child scenarios, the fluid deficit formula is: % Dehydration × Weight (kg) × 10. This tells you how much fluid is required in mL to replace lost fluids, which is then added to their standard daily maintenance fluid total.
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