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Pediatric Weight Estimator

Quickly estimate a child's weight in kilograms from their age.

How it is calculated

The tool picks one of three age bands, with age in years. Under 1 year: weight = 0.5 × months + 4, where months is age × 12. From 1 to 5 years: weight = (age + 4) × 2. Above 5 up to 12 years: weight = age × 3 + 7. The result is rounded to a whole kilogram. Decimal ages are accepted, so 0.5 is six months (7 kg) and 2.5 years is 13 kg. An age below 0 or above 12 shows no result. The bands join with small jumps rather than smoothly, for example 18 kg at 5 years but 24 kg at 5.5. Despite the name, the Broselow length tape is not used: only age is entered, so height and build play no part.

Worked example

A child is 4 years old, which falls in the 1 to 5 year band. Add 4 to the age: 4 + 4 = 8. Multiply by 2: 8 × 2 = 16. The estimated weight is 16 kg. At 2 years the same rule gives 12 kg and at 5 years 18 kg. A child who looks clearly lighter or heavier than the figure should be weighed on a scale, because the estimate reflects age only.

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Common mistakes

  • Entering the age in months. The field is years, so 18 months is typed as 1.5, not 18, and a 6-month-old as 0.5.
  • Entering an age above 12 for a teenager. The estimate covers birth to 12 years only; above that there is no result.
  • Using the estimate as a measured weight for dosing when a scale is available. Age alone misjudges both small and large children.
  • Expecting a smooth curve. The rule switches formula at 1 and at 5 years, so 5 years gives 18 kg but 5.5 years gives 24 kg.
  • Expecting the Broselow tape result. The name points to the same purpose, but this tool does not read the child's length.

Sources

About this calculator

In emergencies where a child can’t be weighed, clinicians estimate weight from age using formulas like the APLS (Advanced Paediatric Life Support) rule: for children aged 1-5 years, weight (kg) is approximately (age in years x 2) + 8; other age-banded formulas cover infants and older children. These are quick approximations, not a substitute for an actual scale reading when one is available.

Estimated weight is critical in pediatric emergency medicine because so much of acute care — drug dosing, defibrillation energy, endotracheal tube size, fluid bolus volume — is calculated per kilogram of body weight, and a child in front of a crash cart usually can’t be weighed first. It’s the same logic behind the color-coded Broselow tape used in ambulances and ERs, which maps a child’s length to an estimated weight band.

A child’s age is the only input; the APLS formula turns it into an estimated weight in kilograms, a fast reference figure for time-critical situations.

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