Why weight matters in paediatric emergencies
Almost every decision in paediatric emergency care is weight-based. This includes adrenaline and amiodarone during resuscitation, fluid boluses, defibrillation energy and tidal volume. The compendium sums it up: "Knowing the weight is half the battle."
Because neither scales nor time are available in an emergency, weight is often estimated from age. This is practical, but it is not a measurement. Anyone using a formula should know where it comes from, where it breaks down and what the alternatives are.
Hierarchy of weight sources
According to the compendium's paediatric emergency table, there is a clear hierarchy:
- Use the actual weight if it is known.
- Ask the parents or carers.
- Consult documents, such as the Austrian mother–child health booklet (Mutter-Kind-Pass) or the German child health record (Vorsorgeheft).
- Use a length-based tape if no weight is known.
- Use an age-based formula as a fallback if only the (approximate) age is known.
The age-based formula therefore comes not at the start but at the end of the chain. It is the tool for situations in which nothing else is available.
The APLS formula: estimating weight from age
The most commonly used age-based formulae come from the Advanced Paediatric Life Support (APLS) course of the Advanced Life Support Group. They have been in use in this form since 2011 and are supported by validation studies [134]:
| Age group | Formula (weight in kg) |
|---|---|
| Infants | 0.5 × age in months + 4 |
| 1–5 years | 2 × age in years + 8 |
| 6–12 years | 3 × age in years + 7 |
The wording of the current APLS edition was not consulted for the compendium. The formulae are supported by the validation literature [134]. Whether older versions of the formula still adequately reflect the weight of today's children has been specifically investigated. This work was part of the discussion that led to the current version [135].
Worked examples from the paediatric emergency table
- Infant, 6 months: 0.5 × 6 + 4 = 7 kg
- 1 year: 2 × 1 + 8 = 10 kg (the infant formula also gives 10 kg at 12 months)
- 5 years: 2 × 5 + 8 = 18 kg
- 6 years: 3 × 6 + 7 = 25 kg
- 10 years: 3 × 10 + 7 = 37 kg
- 12 years: 3 × 12 + 7 = 43 kg
In the compendium, the doses, tracheal tube sizes and defibrillation energies in the paediatric emergency table are based on these estimates.
Limitations of the formulae
1. The formula only knows the average child
Age-based formulae represent typical children. Individual body habitus is not taken into account. The compendium explicitly states that tables and calculated values apply to typical patients. In obesity, cachexia, organ failure, and in preterm and term neonates, individual adjustments are required. A child who is markedly over- or underweight will be systematically misestimated by the formula. The direction and magnitude of the error depend on the individual case.
The evidence base for this article does not provide robust figures on the error range of the formulae. For accuracy in individual cases, reference is made to the validation literature [135]. In practice: a formula is an assumption, not a measurement.
2. Jumps at the age boundaries
The formulae are defined piecewise. When switching from "2 × age + 8" to "3 × age + 7", the estimate jumps from 18 kg (5 years) to 25 kg (6 years). A child does not suddenly become heavier on their sixth birthday. Results in the boundary zone should therefore be checked particularly critically, especially when the age is only approximately known.
3. Uncertain age
The formula is only as good as the stated age. When the age is unknown or estimated, two uncertainties add up. The younger the child, the greater the impact of an error of just a few months.
4. Preterm and term neonates
The age-based formula is unsuitable for preterm and term neonates. The paediatric emergency table lists separate reference values for these groups (preterm neonate 1.5 kg, term neonate 3.5 kg). These do not replace the actual birth weight or current weight. In this group in particular, the difference between the reference value and reality is potentially large.
5. From puberty onwards
The APLS formulae cover children up to 12 years of age. From 13 years or from the onset of puberty, the compendium states that the actual weight should be ascertained. Growth and body composition vary widely at this age, and a linear age-based formula no longer represents this meaningfully.
What an estimation error means for treatment
Weight-based dosing transfers an estimation error linearly to treatment. Overestimating the weight by a quarter means overdosing by a quarter as well. Examples from the paediatric emergency table (sources of individual doses in the compendium, ERC 2025):
- Adrenaline IV/IO: 0.01 mg/kg, max. 1 mg [2]
- Amiodarone IV/IO: 5 mg/kg, max. 300 mg
- Fluid bolus: 10 ml/kg; more cautiously in cardiogenic shock, e.g. 5 ml/kg [2]
- Defibrillation: 4 J/kg, not exceeding the device's adult energy [2]
- Tidal volume: listed in the table as 7 ml/kg
For a six-year-old child with an estimated weight of 25 kg, this gives 0.25 mg adrenaline, 250 ml fluid and 100 J. If the child actually weighs considerably less or more, all values shift simultaneously. A single estimation error thus carries through the entire course of care.
Maximum doses only cap overdosing at the upper end. They do not protect against relative overdosing in a light child whose weight has been overestimated.
In practice: making the estimate safer
- One weight for everything: Agree on a single working weight within the team, state it aloud and document it. All calculations refer to this weight.
- State the source: Communicate clearly whether the weight was measured, reported or estimated by formula.
- Check plausibility: Compare the formula value with the clinical impression. If there is an obvious mismatch, look for an alternative source, such as the length-based tape.
- Adjust early: As soon as a reliable weight is available, update the working weight and adjust ongoing infusions and subsequent doses.
- Use prepared aids: Age-based tables reduce calculation errors under stress. Calculation aids such as those in the "Notfall & Narkose" web app only convert the values entered. They do not remove the need to check the result.
- Follow local protocols: SOPs, care pathways and drug lists of the institution or ambulance service take precedence.
- Check the summary of product characteristics: Concentrations of the preparations used may differ between manufacturers and countries.
Summary
The APLS formula is an established and rapid tool when no weight is known in a paediatric emergency. It provides a starting value, not a measured value. Its weaknesses lie in atypical body habitus, at the age boundaries, with uncertain age, in preterm and term neonates, and from puberty onwards.
Those who know the hierarchy of weight sources treat the formula as a fallback. Agreeing on a single working weight within the team and actively updating it limits the consequences of an estimation error across all weight-based treatment.
References
- [2] Djakow J, Turner NM, Skellett S et al. European Resuscitation Council Guidelines 2025 Paediatric Life Support. Resuscitation 2025;215 Suppl 1:110767.
- [132] Resuscitation Council UK. Paediatric advanced life support algorithm – 2025 Resuscitation Guidelines (Fassungen 10/2025 und V2 11/2025) sowie Paediatric Life Support Guideline 2025, Tabelle 1 „Approximate normal values“ (Wiedergabe in Medscape: Paediatric Life Support – RCUK 2025 Guideline Summary).
- [134] Advanced Life Support Group (ALSG). Advanced Paediatric Life Support (APLS) – A Practical Approach to Emergencies. Altersformeln zur Gewichtsschätzung seit 2011 (Wortlaut der aktuellen Auflage nicht eingesehen; Formeln über Validierungsstudien bestätigt).
- [135] Luscombe M, Owens B. Weight estimation in resuscitation: is the current formula still valid? Arch Dis Child 2007;92:412–415.
This article does not replace clinical judgement in the individual case or the current summary of product characteristics and guidelines.
