How to calculate paediatric weight
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paediatric weight estimation formulas APLS Broselow 2024
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paediatric weight estimation formula accuracy emergency
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PMID: 41579526
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Paediatric Weight Estimation
Accurately estimating a child's weight is essential for medication dosing, fluid resuscitation, and equipment selection - especially when scales are unavailable in emergencies. Multiple methods exist, each with advantages and limitations.
1. Age-Based Formulas
These are the fastest bedside tools but assume normal growth/nutritional status.
APLS (Advanced Paediatric Life Support) Formulas
| Age Group | Formula |
|---|---|
| Infants (1-12 months) | Weight (kg) = (Age in months ÷ 2) + 4 |
| Children 1-5 years | Weight (kg) = (Age in years + 4) × 2 |
| Children 6-12 years | Weight (kg) = (Age in years × 3) + 7 |
| Adolescents >12 years | Weight (kg) = (Age in years × 2) + 8 |
The classic APLS formula "(age + 4) × 2" applies to ages 1-10 years. The updated APLS formula splits into the ranges above for better accuracy.
Nelson/Simple Formula (ages 1-10 years)
- Weight (kg) = (Age in years + 4) × 2
"Best Guess" Method (Best et al.)
- Infants <12 months: (Age in months + 9) ÷ 2
- 1-5 years: 2 × (Age + 5)
- 5-14 years: 4 × Age
2. Length-Based Estimation (Broselow Tape)
The Broselow-Luten Tape is the gold standard length-based tool used in emergency departments worldwide. It is a color-coded tape that:
- Measures the child's body length (crown to heel)
- Uses that measurement to estimate weight
- Also provides equipment sizes (ET tube, laryngoscope) and pre-calculated drug doses per color zone
The American Heart Association PALS course recommends length-based systems. However, the Broselow tape can underestimate or overestimate in populations with high rates of overweight/obesity, and may not be as accurate as newer systems in all populations. - Tintinalli's Emergency Medicine, p. 725
3. Two-Dimensional / Habitus-Based Methods
According to a 2026 systematic review (Fernandez-Aedo et al., Am J Emerg Med) that evaluated 31 studies across all continents, two-dimensional methods are generally more accurate than age-only or one-dimensional formulas, especially where overweight/obesity is prevalent.
- Mercy Method: Uses mid-arm circumference + body habitus score. Does not require age and accounts for body habitus. Shown to outperform standard Broselow in some populations.
- PAWPER Tape (Paediatric Advanced Weight Prediction in the Emergency Room): A length-based tape with habitus adjustment - more accurate than standard Broselow in obese populations.
- Parental estimation: Performs well - parents/carers should always be asked if available.
4. Comparison at a Glance
| Method | Requires | Best Use | Limitation |
|---|---|---|---|
| APLS formula | Age only | Quick mental calculation | Inaccurate in obese/malnourished |
| Broselow tape | Length | ED resuscitation | Less accurate in obese children |
| Mercy method | Arm circumference + habitus | No age available | Requires training |
| PAWPER tape | Length + habitus | Obese populations | Less widely available |
| Parental report | Parent present | Pre-hospital, ward | Not always available |
5. Key Practical Points
- Always weigh the child if possible - estimated weights carry meaningful error across all methods
- In emergencies where weighing is impossible, use a length-based tool (Broselow/PAWPER) as first choice, combined with habitus assessment
- For drug dosing, cap estimated weights at clinically appropriate limits (e.g., don't dose a morbidly obese child as if they were a lean adult)
- The systematic review recommends that each emergency system validate methods locally to suit their demographic profile, as accuracy varies significantly by population
Recent evidence note: Fernandez-Aedo et al. (2026, PMID: 41579526) - systematic review of 31 global studies - confirms multi-dimensional methods outperform single-variable formulas, and recommends local validation before adopting any single approach as routine practice.
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