From the Research
The most accurate formula for approximating weight in children is the Best Guess formula, which varies by age group, as it has been shown to have the smallest overall mean difference in a study of 49,565 patients 1.
Age-Based Weight Estimation
For infants less than one year, the most accurate formula is the new APLS formula, while for children over one year, the Best Guess formula should be used 1.
- The Best Guess formula has been validated in a study of 1843 patient weights and has been shown to more accurately estimate weight than both APLS and ARC formulae for preschool-aged and school-aged children 2.
- The formula for infants under 12 months is: Weight (kg) = (age in months + 9)/2, and for children aged 1-5 years and 11-14 years, the Best Guess formula is the most accurate, with mean differences of 0.27 and 0.20 kg respectively 1, 3.
- For children aged 6-10 years, the updated APLS formula is the most accurate, with a mean difference of 0.42 kg 1.
Limitations and Considerations
- These formulas provide quick estimates in clinical settings when actual weight measurement isn't possible and are particularly useful in emergency situations for medication dosing calculations.
- However, they are only approximations and can have an error margin of 10-20% 4.
- Growth charts remain the gold standard for assessing appropriate weight for age, and factors like genetics, nutrition, and underlying medical conditions can cause significant variations from these estimates 3.
- When possible, actual weight measurement is always preferred for accurate clinical decision-making.