For infants under 1 year, enter age as a decimal (e.g. 0.5 for 6 months) or use months directly below.
How it's calculated
These formulas estimate weight from age only, for use when a scale isn't available (e.g. a prehospital emergency) — a measured weight, or a length-based tape (e.g. Broselow), should always be used in preference when available.
Formulas used
| Age range | Formula |
|---|---|
| 0-12 months | Weight (kg) = (age in months ÷ 2) + 4 |
| 1-11 years | Luscombe-Owens: Weight (kg) = (age in years × 3) + 7 |
| 1-10 years | Classic formula: Weight (kg) = (age in years + 4) × 2 |
Clinical use
- Used for rapid weight-based drug dosing in emergencies when the child cannot be weighed (e.g. cardiac arrest, prehospital care).
- The Luscombe-Owens formula was derived from more recent population data and is generally considered more accurate than the older "classic" formula, which tends to underestimate weight in many modern populations.
- A length-based system (e.g. a Broselow tape) is generally preferred over an age-based formula when available, since it doesn't assume a "typical" child for that age.
Frequently asked questions
Why are there multiple formulas shown?
Different formulas were derived from different population datasets and eras. The Luscombe-Owens formula (2007) is based on more contemporary data and is generally preferred; the classic (age+4)×2 formula is older but still widely taught and used.
Is an age-based formula as accurate as a length-based tape?
Generally no — length/height-based systems (like a Broselow tape) tend to estimate weight more accurately than age alone, since they account for a child's actual size rather than assuming an average for their age. Use a length-based method when available.
Should this replace an actual measured weight?
No — these are estimates for when a scale genuinely isn't available. A measured weight should always be used in preference for drug dosing whenever possible.
References
- Luscombe M, Owens B. Weight estimation in resuscitation: is the current formula still valid? Arch Dis Child. 2007;92:412-415.
- Advanced Life Support Group. Advanced Paediatric Life Support: The Practical Approach.