Why estimate a child’s weight?
In children, almost every emergency dose, from fluids to medicines to defibrillation energy, is calculated from body weight. In a resuscitation, there is often no time or no way to weigh a child, and parents may not know a recent weight. Age-based formulas give a quick starting estimate.
The APLS formulas
The Advanced Paediatric Life Support (APLS) course uses three formulas, updated in 2011 to fit modern growth data:
| Age | Estimated weight (kg) |
|---|---|
| 0 to 12 months | 0.5 × age in months + 4 |
| 1 to 5 years | 2 × age in years + 8 |
| 6 to 12 years | 3 × age in years + 7 |
The older formula, (age + 4) × 2, is still taught in places but tends to underestimate the weight of today’s school-age children.
Worked examples
| Child | APLS estimate | Old formula |
|---|---|---|
| 6 months | 7 kg | not applicable |
| 4 years | 16 kg (about 35 lb) | 16 kg |
| 8 years | 31 kg | 24 kg |
For the 4-year-old, the calculator also shows weight-based reference values often used in pediatric emergencies:
| Reference | Value |
|---|---|
| Fluid bolus at 10 mL/kg | 160 mL |
| Defibrillation at 4 J/kg | 64 J |
How accurate are these formulas?
Not very, for any single child. Studies find that age-based estimates can miss the true weight by 20% or more, and they tend to underestimate weight in populations with high rates of obesity. Better options, when available, are:
- a recent measured weight, for example from the child’s health records;
- a length-based tape, such as the Broselow tape, which reads the weight from the child’s length;
- the parent’s estimate, which is often reasonably accurate.
Doses for children with obesity may need to be based on ideal or lean body weight rather than actual weight, depending on the drug.
Not for home dosing
This calculator is designed to illustrate emergency estimates. For over-the-counter medicines such as acetaminophen or ibuprofen, parents should use the child’s actual weight, or the age bands on the package, and ask a pharmacist or pediatrician when unsure.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.