# Paediatric Medication Calculation

> Nursing notes on paediatric dose calculation: mg per kg dosing, Dilling Young Clark rules, 4-2-1 fluids, ETT and defibrillation formulas with double checks.

- Canonical URL: https://prepelephant.com/topics/allied/nursing/paediatric-medication-calculation
- Exam / course: Allied Health · Subject: Nursing
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Paediatric Medication Calculation", PrepElephant, https://prepelephant.com/topics/allied/nursing/paediatric-medication-calculation

## Direct answer

Fourteen kilograms of toddler, paracetamol ordered at 15 mg per kg — the arithmetic (14 × 15 = 210 mg) is the easy half of paediatric medication safety; the hard half is weight accuracy, unit discipline (micrograms versus milligrams) and the independent double-check that catches the decimal slip before it reaches a child. Most paediatric prescribing is weight-based (dose per kg per day divided into frequencies), with age-based formulas — Dilling's (age divided by 20 times the adult dose), Young's (age divided by age plus 12), Clark's (weight in pounds divided by 150) — retained for exams and resource-poor settings. Resuscitation runs on formulas worth memorising: maintenance fluids by the 4-2-1 rule, endotracheal tube size (age in years divided by 4, plus 4 uncuffed), defibrillation 2 then 4 joules per kg, adrenaline 0.01 mg per kg, fluid boluses 20 mL per kg.

## What you must remember

- **Weight-based core:** dose = ordered mg per kg × weight in kg, checked against frequency — paracetamol 15 mg per kg per dose every 4 to 6 hours is the classic example, amoxicillin 40 to 90 mg per kg per day in divided doses; re-read the per-day versus per-dose wording.
- **Age rules for adult-dose fractions:** Dilling's — age divided by 20 (the Indian textbook favourite); Young's — age divided by (age + 12); Clark's — weight in pounds divided by 150.
- **Body surface area:** from a nomogram or Mosteller's formula (square root of height in cm × weight in kg divided by 3600); chemotherapy doses as mg per metre squared.
- **Maintenance fluids (Holliday-Segar 4-2-1 rule):** 4 mL per kg per hour for the first 10 kg, 2 for the next 10, 1 for every kg above 20 — a 26 kg child needs 66 mL per hour (40 + 20 + 6).
- **Resuscitation formulas:** uncuffed ETT internal diameter = (age in years ÷ 4) + 4, cuffed + 3.5; adrenaline in arrest 0.01 mg per kg IV (0.1 mL per kg of 1 in 10,000); defibrillation 2 then 4 J per kg; crystalloid bolus 20 mL per kg.
- **Weight estimation without scales:** APLS formula — (age in years + 4) × 2 for 1 to 10 years; Broselow tape by length in emergencies.
- **Safety rituals:** an independent double-check for every high-alert drug (insulin, heparin, opioids, chemotherapy), pump rates verified against the order, never a trailing zero (5 mg, not 5.0 mg) nor a naked decimal (0.5 mg, not .5 mg).

## A ward calculation from prescription to pump

A 6-year-old, 20 kg, is ordered ceftriaxone 80 mg per kg per day IV in two doses for pneumonia. The calculation: 80 × 20 = 1600 mg per day, so 800 mg per dose. The vial is 1 g in 10 mL of reconstituted solution (100 mg per mL), so the nurse draws 8 mL. Before pushing, the second-nurse check walks the chain aloud — today's weight (not last week's), per-day versus per-dose wording, units, concentration, final volume — because each link is where errors live. For an infusion the arithmetic extends: vancomycin 60 mg in 50 mL over 60 minutes programmes as 50 mL per hour; a 4-2-1 check confirms maintenance at 60 mL per hour for this child. In the emergency corner the discipline compresses: a seizing 16 kg child needs lorazepam 0.1 mg per kg — 1.6 mg — drawn as 0.32 mL of the 2 mg per mL ampoule; a second nurse confirms while the clock runs. Every step is signed.

## Where Indian exams catch candidates

Paper questions repeatedly test Dilling's rule (used widely in Indian textbooks) against Young's and Clark's — knowing which formula divides by 20 separates prepared candidates from guessing ones. The 4-2-1 rule appears both as a sum and as a trap (the second 10 kg runs at 2 mL, not 4). Error-pattern questions quote a medication incident and ask what failed — pounds read as kilograms, "per day" given as a single dose, a decimal shift — expecting "independent double-check and unit discipline". Vivas probe drops per minute with a microset (60 drops per mL) and converting mg per kg per hour orders into pump settings. The 4-2-1 logic underpins WHO/IMNCI rehydration planning, and Indian protocols mandate double-checks for high-alert paediatric infusions — say "double-check" early and often.

## Frequently asked questions

### Which age-based formulas estimate a child's dose from the adult dose?

Dilling's (age ÷ 20), Young's (age ÷ age + 12) and Clark's (weight in pounds ÷ 150) — Dilling's is the commonest in Indian textbooks.

### How is the 4-2-1 rule used for maintenance fluids?

4 mL per kg per hour for the first 10 kg, 2 for the next 10, then 1 above 20 kg — summed for the hourly rate, with half given as dextrose-saline per protocol.

### What size endotracheal tube fits a 4-year-old?

Uncuffed (4 ÷ 4) + 4 = 5.0 mm; cuffed 4.5 mm — confirmed by leak.

### What are the paediatric defibrillation and fluid bolus doses?

Defibrillate at 2 joules per kg, then 4 for subsequent shocks; give crystalloid boluses of 20 mL per kg, reassessing after each.

### Why is an independent double-check mandatory for paediatric high-alert drugs?

Because mg-per-kg arithmetic, decimal shifts and dilutions multiply error opportunities, and a second nurse checking the calculation, vial and pump independently intercepts most of them.